Minimost XL modifications

[randyt]

Anybody taking their engine in to a dealer for a tune-up/check-up should ask for the full-throttle rpm figure for it, as a target to shoot for when trying props.  Write down the lower unit gear ratio so you know what the propeller rpm should be.  See if they have a prop chart for the motor, listing the range of props covering both big, heavily-loaded boats and lightweight fleas.  Ask if there's a local prop expert.

And make sure they lube the lower unit.

All good tips . I have the ratio, 2.41 and full throttle is 5400 from what i have read so far but will double check . I will see if he has a prop chart , the only one I found is rather generic for " normal " boats . Yes we have a prop tuner locally, https://www.halepropeller.com/ .  Chris rebuilt the lower unit for me , changed out the impeller all ready . the motor starts and runs but we are mostly looking for him to perhaps rebuild the carbs if needed and give it a good looking over . I was thinking of having him put in a more advanced reed system but we decided to get it running good in stock configuration for now and proceed with the boat tune before trying to improve the motor.

Thanks again for the tips, greatly appreciated.

[Viky_CZ]

Am I right to think that 2.41 ratio is way too slow for speedboat?

[seattle smitty]

Well, you are right in thinking that that is a lot of "underdrive" by speedboat standards. 2.41:1, meaning the crankshaft makes 2.41 revolutions for every one turn of the propshaft, IS quite a lot of under-drive for a little high-speed boat  .   .   .  which suggests that you need a prop with more pitch, maybe a lot more pitch, than the standard fishing propeller to make your little boats GO.  I raced with gear ratios like 12:15 underdrive, 16:21 underdrive, even 1:1 "direct drive."  And this was with crankshafts turning, depending on engine, peak revs anywhere from 7100rpm (stock racing Merc Mk20H)(avatar photo on left) to 10,000-some rpm (250cc Konig on methanol).  When you see gear figures like the 16:21 above, that meant the upper (driveshaft) bevel gear in the racing lower unit had 16 teeth and the lower (propshaft) bevel gear had 21 teeth, so if the crankshaft/driveshaft was turning exactly 7100rpm, the propeller shaft was turning 5408rpm.  The 1:1 ratio (which I also used on the boat at left) was with bevel gears that each had 14 teeth, IIRC) had the propshaft turning the same 7100 rpm as the engine, but with this ratio one used much smaller propellors, so the boat ran about the same speed.  The choice between the two ratios in this particular class usually hinged on what props you had.  If you had a really good 1:1 prop, and mediocre 16:21 props, well, you see the choice.  One more mile per hour was and is a big deal in Stock racing. 

I offer all this old racing lore just so you get a flavor of what gear ratios and prop choices can mean when you are working for the last little improvements.  This process is time-consuming and expensive, and certainly is not needed or even desirable with fleas. But it points in a general direction for making a boat work better.

[rogerduncan100]

I doubt there are any prop sellers in Southern Ontario who will let you try out a whole whack of their props and then let you pick the best one. Otherwise, one must buy the prop before they get to try it. Expensive indeed.

[seattle smitty]

So what you do, instead of just buying props on the chance they'll work, is first to write down your engine make, model, year, rpm and gear ratio, take a few photos of your boat out of the water with the engine hung on the transom, and email that to a good prop man like Ron Hill and ask him where to start. Tell him what you weigh, or at least guess at the anticipated load, and tell him the weight of the boat and the width of the bottom. He might have something that will work, or maybe he can suggest modifications to a prop you have.  Explain to him that you're not doing serious racing, and don't want to sink a bunch of money getting the last mile an hour.

Used props can sometimes be found gathering dust on marina shelves, or on Ebay or Craigslist, so you don't necessarily have to buy new.


We have something new to look forward to here shortly.  Randy, who has considerable experience in racing sail and knows the performance value of adding up incremental improvements arrived at with systematic testing (as opposed to seat-of-the-pants guessing), will be describing his boat-tuning sequence, I say his FULLY-INSTRUMENTED boat-tuning sequence, with comparative readings of ACTUAL rpm and mph with changes in motor set-up, props, and so on.

[randyt]

It will be a interesting experiment and I will have two months to get it sorted out and then my summer vacation is over . I will post what I can and will entertain all suggestions and comments . Smitty has given me some good guidance thru the building and modification phase so far and its been a big help . Approaching a good prop man once I have my facts straight is certainly on the list but I would like to see just how far off I am before I do that . Once I am in the water and get those base readings I will have a much clearer picture as to how to proceed or at least have more educated questions I received a few more photos today and its only making the wait harder .

[randyt]

Well after being connected to the project only by emails and sending checks for 8 months I finally am in CT with the boat and we are beginning our experimenting .

First day was just with the boat au natural, no trim appendages just to basic hull. The lake was calm , 90% full tank of fuel in the bow tank.

The boat got on a plane easily but did NOT like to go slow . It was porpoising quite badly in the slow to mid range stages . As we got into the higher RPM range about 4,000 the boat started to smooth off. All of this was with the motor in the fully trimmed in position . As we began to trim it out the boat freed up but it was a fine line and began to both bounce a bit and on the up move we could actually feel the rear start to come free of the water a bit so that was our limit, Speed was about 34 MPH with a max of 5100 RPM. This was with out 12 1/4 - 17 Michigan Wheel aluminum prop.  To be honest i was a bit concerned because I am actually planning on spending more time cruising than actual top speed runs . The steering was much improved over the previous motor , the Nissan 40 . I knew we had a long way to go but was happy with the apparent reliability of our refurbished motor. There is a slight chance that we are not getting full throttle out of it and will be having that looked into in the near future.  So the end result of day one was we feel very good about the hull strength wise and the modified transom was doubling as a nice water break as surprisingly enough we had NO water in the boat after a couple hrs on the water .

Day Two

Lake was calm, just under a half tank and Nauta smart tabs installed .

We took off with the tabs disengaged to get our selves re acquainted with the boat for one lap. I had purchased the optional bracket that allows me to just throw a lever and it lifts the tabs clear of the water which I wanted to be able to do with our testing and its a nice option if you are pulling up on a beach etc.

We engaged the tabs to their mid position and all I can say is that it is a different boat. Instead of taking a boat length and a half perhaps to get in plane, which is good obviously , its on plane in just about 10 ft which is of course better . The really big difference and one which gave me a good bit of relief as mid range speed was smooth and controllable down to 14 MPH . The steering was also improved a good deal . This boat really likes to lean into a turn and take a set as we say in the car world. I feel pretty good about the safety of a tight turn as the shape of the chines are at a angle which will be less apt to trip . When you press too hard peed the back end broke loose a little bit but it was quite controllable with good warning and feedback.  Top speed was about 36 at 5100 RPM.

Now for the confusing part . With the trim tabs the boat did not like any lift at all . The hop would begin almost as soon as you started to trim it out . The boat would of course begin to speed up but it would hop as well. I was not able to get anywhere like i did the first day when you could actually feel the back end break free just a bit. I've only seen two other Minimost XL's like mine and they both are run in a very negative trim position , is it possible that with the design of these hulls its just the way it is ? Of course with the hull making so much contact with the water its also a wet ride as well as slower than I think it could be .

One other difference between the days was that now we were down to a quarter tank of fuel so factor in perhaps 70 lbs less in the bow.  SO in one way I am going in the correct direction for getting the boat to cruise a bit but the fact I cant even begin to trim it out is a bit disappointing , I know the game is new but its still confusing.

Now our plan is to fill the tank and go back out again and see what we can get out of the trim tabs , Then we have another item to try,  the hydrafoil . I know that the trimming elements create drag but our hope was that we had enough power with the 50 to over come that and still get some speed out of it .

Anyway it has been a good week , when i got here the carbs were out for a rebuild and neither the boat nor trailer was registered and all that is done and we have a running boat . We plan on getting back out in a day or so and I will update accordingly .  Ive included a few pics . It is odd but if you youtube search MinimostXL they all have the same bow down attitude so it might just be what I will live with but i'm not giving up yet

[rogerduncan100]

For my comment I am assuming that when you say the boat had a "hop" you are referring to the boat porpoising. You need to move the centre of hydrodynamic lift further aft to get rid of the porpoising. It needs to be close to or behind the centre of gravity which is itself quite far aft in this boat. I think the easiest way to accomplish this is to make the trim tabs two or three times bigger. From the pictures it appears there is space to do this.

http://muskokaseaflea.ca/bb/index.php?topic=1629.msg10952#msg10952

All the best and have fun with your setting up!

[randyt]

Thanks for the comments. As the trim tabs are set in the middle posistion if I increase the leverage and make them “ work “ harder would that help move the lift you refer to rearward at all ?  Will the add of the additional fin the the antiventalation plate help in this regard in any way ?

Thanks again !

[rogerduncan100]

I thought already of the idea of trimming the trim tabs down (so they bite more). It would have some effect but with a drop in top speed. It's quite similar to having a hook in the keel, like the one quarter inch "monster" my boat, Suzie, has. Hooks are a problem because they cause the centre of hydrodynamic pressure to move fore and aft a lot as the angle attack of the boat changes.

I think it's much better to run the trim tabs flat, not biting. Making them bigger would bring the centre of lift aft where it needs to be.

Putting a large fin (like in the last picture you posted) on the antiventilation plate is probably going to be the best answer to your problem. You'll have to be sure to trim the motor correctly. Ideally, the fin should be coplanar with the bottom or possibly "kicked in" a little bit, but not too much. If you have power trim it's an easy adjustment to make. The large fin will lower top speed a bit but life is full of compromises. 

[randyt]

Thankfully we do have a nicely functioning power trim which is the reason we went up in size to this particular motor as the Nissan 40 that came with the boat did not have this option . We will be trying the large add on fin shortly and I will report our findings . As you say is all a trade off . More drag better ride perhaps but at a loss of top speed . I will keep the reports coming .

[seattle smitty]

Good report, Randy.  I'm guessing the whale-tail hung on the lower unit will work better than the tabs (subject to finding the right motor-height and angle).  Eager to hear about it.

[randyt]

Ok so the next installment on our set up process is complete.

We installed the Hydro fin and as I am becoming used to the results were good but not what i expected.

I was able to finally be able to use the trim again and the take off is almost laughable as there is almost zero bow lift, the boat just rises up a bit and then goes flat.

With the trim tabs in their least powerful mode the boat was able to achieve the same speed as without the fin as I was able to induce a few clicks of positive trim on the motor. We could get it to 35 nice and steady but with one more click it would hit 36 but the proposing would begin. Once click back down and it was steady again , that was its sweet spot. The RPMs were again at the 5000 mark. BTW according the the chart my desired rev range is from 4500 to 5500 RPM.

So the boat is steady , turns well and is controllable so I would say the basics are covered but now its the fine tuning we can play with .

We were able to get down to the 12 MPH range and still stay on plane which I was looking for which would be desirable for just cruising into a new area which was impossible before I started adding the trim items .

As an experiment we put the trim tabs in full attack mode and of course the boat got up on plane very well and amazingly enough we were still able to hit our 35 mph range . What was really nice was that I could get the boat to motor along at 5 mph with the bow down which was something I could not do before so I think that will also be useful in spots I am unfamiliar with.

Now for the negative of all this trim capability . We happened to come across a wake by surprise and buried our flat little platypus nose and shot a huge wall of water straight up into the air which we then passed thru and we did it to the second wave as well . Other than getting quite wet and the laughter subsided we decided we better work on some sort of evasive maneuvers for this type of situation .   Here is the rub. If , for example , we are moving along at 25 mph and spot a wake it takes us at least 15 seconds to slow down enough that the transom will sink a bit and the nose will come up . Between the " speed " of the trim and the natural slowing of the boat we cant seem to get into " safe mode " any quicker than that. We tried an abrupt pull back on the throttle , which is risky with a following wake but that didn't do anything at all . With all of our add ones to get the boat flat we can't get the nose up at all when we need to . Our only answer at the moment is to keep an eye peeled up far ahead and slow down far in advance of any trouble. We will work more on technique and see what other maneuvers we can try .

That is where we are at present and things are improving with each trip out. We noticed that our volt meter was pegged at 16 volts the time before and we replaced the regulator and all is good now with no more than 13 volts being recorded at any time.

I will keep posting when i have new info .

Randy

[rogerduncan100]

Nice going, it looks like you've got things set up pretty well now. I can't figure out how you might slow down quicker without defying the laws of physics unless you pioneer some sort of boat brake.

Maybe you could squeeze out more speed with a higher pitched prop? Possibly, but I have heard this rule of thumb: each inch of pitch lowers RPM by 2000. Even with your flat-bottomed beauty the rule probably applies to some extent.

[seattle smitty]

Now for the negative of all this trim capability . We happened to come across a wake by surprise and buried our flat little platypus nose and shot a huge wall of water straight up into the air which we then passed thru and we did it to the second wave as well . Other than getting quite wet and the laughter subsided we decided we better work on some sort of evasive maneuvers for this type of situation .   Here is the rub. If , for example , we are moving along at 25 mph and spot a wake it takes us at least 15 seconds to slow down enough that the transom will sink a bit and the nose will come up . Between the " speed " of the trim and the natural slowing of the boat we cant seem to get into " safe mode " any quicker than that. We tried an abrupt pull back on the throttle , which is risky with a following wake but that didn't do anything at all . With all of our add ones to get the boat flat we can't get the nose up at all when we need to . Our only answer at the moment is to keep an eye peeled up far ahead and slow down far in advance of any trouble. We will work more on technique and see what other maneuvers we can try .    Randy

That seems odd. When you suddenly back the throttle down, the slow-turning prop should create a lot of drag.  It's like the prop is free-spinning or something (How? Impossible as far as I know.).  You say you can adjust the tabs so that you don't need the whale-tail?  Could the tabs be made operator-adjustable while underway?  And if so, would raising the tabs quickly at the same time you pull back the throttle tend to let the bow rise? I believe some of the Inboard racing runabouts have a foot-pedal that lets the driver adjust the big cavitation plate while at speed (probably with a rod that rotates a camshaft)(I haven't been to an Inboard race in a long time).

These accounts of your testing regime are very interesting, Randy.

FWIW, water-brakes on boats have been done, using a pneumatic cylinder that shoves a length of rod into the water, doesn't have to be very big, either, at higher speeds.  I think they powered these with something like a Freon cylinder (used for "air"-horns), and a spring retracted the rod. Such a brake could be made to slow the boat, though it would also have the unwanted (in this case) effect of pulling the bow downward.

I'm glad to hear the boat has shown enough recovery moment (plus all that weight on the transom) to bring the bow back up when you've dived into a roller.  Bad news if it keeps diving under!!

Roger, remember your rule of thumb on pitch goes out the window if you are willing to play with reducing blade area with the higher pitch props.