Author Topic: 35 Evinrude on Minimost  (Read 305 times)

rogerduncan100

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35 Evinrude on Minimost
« on: September 10, 2018, 01:31:57 AM »
I'll be trying a seventeen inch pitch prop on my Minimost in due course. Theoretically it should yield over 40mph at 6000rpm for my '79 35Evinrude if I remember correctly. Should be interesting.
Roger, aka rogerduncan100

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #1 on: September 15, 2018, 11:31:44 PM »
Once I tilted the motor down I was able to run the setup without it becoming squirrely at high speeds. I was surprised the motor wouldn't turn faster on such a light small boat. It looks like it was turning only 5000 rpm if that. With an eleven inch prop on my 13 ft runabout this same motor turns between 5700 and 5800. On this same boat the seventeen inch prop turned only 4100 rpm.

The motor is turning faster on the smaller boat but not as much as I had hoped.

A couple of things I will try are raising the motor so the cavitation plate is above the bottom of the boat and adding bite to the trim tabs so I can run the motor at the second tilt, just slightly kicked in. 
Roger, aka rogerduncan100

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #2 on: September 16, 2018, 03:24:59 PM »
Top speed was only 33 mph, though I have to say that was enough to focus my attention. Here's a picture of the motor on the boat kicked in so it wouldn't go squirrely at high speeds. Definitely not a long term solution!
Roger, aka rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #3 on: September 18, 2018, 06:11:55 PM »
Well, I'll be the first of your legions of fans to check in, Roger.  The existence of this sub-forum surely has more to do with your own on-going reports than anything else.  Very good!!

I must say that setting up one of these very short boats for best ride, minimal porpoising, good handling, and so on, while carrying a rather heavy engine, has got to be a trickier task than setting up raceboats, which are longer, have engines which are much lighter in relation to boat size, and which have had a very long development history, with set-up proceedures well and widely known among the owners.  So the prop and set-up experiences you describe here should fill an information void that racers don't have to deal with.

Regarding props, I see some flea-owners here have been disappointed when they have tried higher-pitch wheels on their fleas.  I've tried to stress that for getting more speed out of a light boat, most commonly what will work best is not only more pitch than the factory-original prop, but also a reduction in blade area to help the engine wind out to its best rpm for wide-open throttle.  If you can get it, a two-blade will most likely be better in this regard than a 3-blade.  A round-ish blade shape will be much better than any weedless fishing prop design, of course.  But given that, too much blade area could keep your engine below its proper operating rpm.  You might well be uneasy about cutting down the blades of a prop you have had difficulty finding in the first place, but if it isn't working out as hoped, well  .  .  .  .

If you are anywhere near where small outboard hydros and runabouts are raced (not big ski-boats, bass-boats, or tunnel boats, you could visit a race, propellors and a felt-pen in hand, and talk to the OLD experienced racers who were racing in the days of round-blade props.  Describe your problem, and see if they will mark out a good reduced blade shape on your prop with the felt-pen.  I suspect that they will draw a line that removes more from the leading and trailing edges than from diameter.  At home, take a deep breath and make the cuts; after you finish shaing the first blade, which was marked with the felt pen, you'll have to trace around that blade to make a pattern of your own devising to transfer the shape to the other blades. Then re-sharpen.  Get the blades as close to each other in shape and thickness as you can, but they won't have to be microscopically identical. The prop does have to be balanced, so find a machine shop, or auto engine shop that can do a simple static balance.

A more reliable way, but one which would cost some money, would be to get a good racing prop-man to do the whole job based on your detailed description.  I have mentioned Ron Hill in California as one of the most widely experienced, but there are many others.
« Last Edit: September 19, 2018, 05:12:35 PM by seattle smitty »

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #4 on: September 19, 2018, 03:19:40 PM »
Yes, Smitty, my newly acquired 17 inch prop for my 35 seriously reduced the rpm's so much so that I have to wonder what use the prop would be on any boat at all. Fortunately my new prop has already "broken". I accidentally scraped it on the driveway while pushing the boat and trailer and a sizeable chip (no doubt part of a repair) detached. At some point I will take a saw and file to this prop and see what I can do but in the mean time I will use the 11 inch prop I got with the motor.

What I want to do is get rid of the porpoising if I can. I will adjust the trim tabs for more bite to compensate for the heavier motor. The way I understand it, porpoising is caused when the centre of gravity and centre of hydrodynamic lift are not closely aligned on the fore-aft axis. By increasing the bite of the trim tabs I will move the centre of lift aft to match the aft movement of the centre of gravity I caused when I added a heavier motor to the setup.

This will be a lot easier when I visit my brother at his cottage on Stony Lake rather than trying to do this at Bluffer's Park in Scarborough, fighting the Lake Ontario waves all the while.     
Roger, aka rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #5 on: September 28, 2018, 01:50:26 AM »
I did a test run on Lake Ontario today and there was no porpoising of the boat. That's because I added bite to the trim tabs. I was running on the second tilt which is the right one. The engine is very slightly kicked in.

However, there was instability at high speeds.

I'm hoping the instability was due to the excess bite on the trim tabs. I will do a test with the trim tabs set with a bit less bite.

The other thing I am thinking about is raising the engine so the cavitation plate is above the bottom of the boat. That could have a big effect, I think.
Roger, aka rogerduncan100

EDSRX3

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Re: 35 Evinrude on Minimost
« Reply #6 on: September 30, 2018, 09:52:25 PM »
Keep trying the 17.  As you dial it in the revs will come up.

I'm running the 10 x 17 with the 35 rude on the minmax. Currently stock.  I'll see what happens with the new bottom.

Right now the tach runs around 5600 and hits 6000 in the sweet spot.  It will bounce up to mid 6's if it grabs some air.

As you lift the motor the tilt will probably need an adjustment.  Eventually you will start grabing air in the prop.  Too much is a problem especially on turns. A little let's the prop slip, like slipping the clutch.  Brings up the revs and speed, as long as the prop eventually clears the air( which may not happen and then you back up the last change or try something else). 

I start carving up a prop after I try everything else.

Ed

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #7 on: October 05, 2018, 02:41:29 AM »
I had some good runs on Stoney Lake.

I reduced the bite on the trim tabs and that reduced instability at high speed but caused more porpoising at slow planing speed, but not enough to be a problem - definitely a good trade off.

The eleven inch prop had way too little pitch causing severe over-revving and scary hole shots where I thought I might go backwards over the motor. The trimmed seventeen inch prop worked very well - decent, safe hole shots and stability at high speeds.

I also raised the motor but think raising it more might be even better. The cavitation plate is just at the level of the bottom right now and throws out a lot of spray to each side.

I'm happy with the speed and rpm's - 37 mph at 5600 rpm for a slip of 20% (80% efficiency). Maybe more dialing will get me 40 mph.

Pictures show my temporary tach-speedo mount, the boat and the props side by side the same distance from the camera. 


« Last Edit: October 05, 2018, 02:44:01 AM by rogerduncan100 »
Roger, aka rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #8 on: October 05, 2018, 11:04:23 AM »
Good photos.  My stars, that is a big engine on that little boat  :-o  Anyway, I'd guess you have another inch or so of shims you could try under the engine's clamp bracket, say 3/8 to 1/2" at a time while watching the tach.  But you'll have an additional effect, in that right now the motor's "cavitation plate" may well be having a big effect on the ride, amounting to a third trim-tab (and something utterly unknown to racers).  So if you get the motor up more and gain speed (and still are getting enough water to the engine) but the porpoising worsens, and can't be fixed with motor angle, here's where one of those "whale tails" might possibly be of value.  I'd think you could cut a small one out of 1/8" aluminum with a saber saw, drill some holes, and bolt it to the bottom of that cavitation plate to add some area to it.

Too bad you dinged the prop, but it sounds like your trimming of the blades worked out well enough, given that your rpms must be near what I imagine you want.  Sounds like if you raise the motor, you may find you could handle even some more pitch.  Or some cup.  I doubt that your cast aluminum prop would stand up to any re-pitching or cupping, unfortunately. So if you get the set-up dialed-in, and you need more speed with no more revs, you could watch for an 18" (or more) prop.  If none fits directly, maybe you can adapt one from another engine, maybe by boring out the hub and machining an insert that fits it to your propshaft.

Ain't this fun?!! :-D

This thread is exactly what I had hoped to see in this new sub-forum.  Great story, Roger!!
« Last Edit: October 05, 2018, 11:09:10 AM by seattle smitty »

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #9 on: October 11, 2018, 02:21:14 PM »
I can't believe it, my internet crapped out a day and a half ago and I'm in a doughnut shop. It's probably the last warm day in Toronto too.

I have the motor jacked up really high with the cavitation plate about an inch or a bit more above the bottom. It's hard to keep the prop gripped on the water. If I ever find some flat water I'm thinking I'll be able to beat my personal 37mph speed record.
Roger, aka rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #10 on: October 13, 2018, 04:20:38 PM »
The waves are too big on Lake Ontario these days and the air is too cold. I might be waiting until spring to get flat water to try the motor the way it's jacked up now.
Roger, aka rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #11 on: October 14, 2018, 03:00:19 AM »
Man, it looks really jacked up now.  Can you tell that the engine is still getting enough water? Anyway, I'm gonna miss this ongoing story, Roger. Hope y'all have a good winter.

rogerduncan100

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Re: 35 Evinrude on Minimost
« Reply #12 on: October 14, 2018, 01:24:24 PM »
Yup, looks like I might be waiting six months or so to try to break my record. The motor does seem to get enough water, lots coming out when I check the pee stream. The water intake is almost completely below the bottom level of the boat. Winter's on the way, time for more inside playtime.
Roger, aka rogerduncan100