effect of trim on speed

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james__12345

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I have a 16 foot lonestar mod v aluminum boat, with an 1968 18 horse evinrude on it. According to the formulas I've found for determining prop slip the theoretical top speed for this motor is a little over 24 mph. I'm only getting about 17 out of it. I also read that 10 percent slip is a decent expectation but mine is closer to 30 percent.

The prop is brand new, the motor is running about 4400 rpm. I have noticed that the trim isn't quite right. Even using the furthest out hole on the motor, it is still trimmed in quite a bit too much. If I had to guess the motor needs to go out so that she leg is about 3 inches further away from the hull, at the very bottom of the hull. My question is how much of a difference would getting the trim set right make in my speed? I'm not sure how to go about fixing it just yet, but I'd like to have some idea how much difference it would make before I go through the trouble.
 
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82rude

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Re: effect of trim on speed

james,i believe your just asking too much from that little motor on such a boat.whats the rating of that boat?i went on line and all the lonestars I found in that length had 40 to 55 hp. motors.a general rule of thumb is 70 percent min of stated hp rating.youre over 50 percent I believe,unless your lonestar is somehow dractically different then the ones I seen.
 
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james__12345

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Re: effect of trim on speed

I dont know the rating. Its not on the boat and I cant find my boats information online anywhere. I've spent hours and hours trying to find the info on it. I found a list of what is supposed to be every boat they made, but mine isn't on it. Mine is quite a bit skinnier than any of the 16's I can find listed.
 
Sea Rider

Sea Rider

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Re: effect of trim on speed

That 16" footer is rated for at least a 25 HP engine, If wanting to get the max out of that combo, float you boat alone, set trim hole to achieve a perpendicular 90? engine to water level; with well ballanced boat at wot, boat should ride near paralell to water surface. If engine sits at best transom height with no back or over transom splashes whatoever, just near flat wake out engine, the only thing left without installing a higher engine HP, would be to optimize engine rpm. Top speed should better slightly.

Were 4400 revs measured with a tach, if so, a seems very low. Check min-max wot rpm factory working parameters and select a prop that gives at least middle to near fully max rpm at wot. You definitely are lugging engine at 4400 need -less prop pitches to increase engine revs.

Happy Boating
 
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james__12345

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Re: effect of trim on speed

Sea Rider said:
That 16" footer is rated for at least a 25 HP engine, If wanting to get the max out of that combo, float you boat alone, set trim hole to achieve a perpendicular engine to water level (90?) with well ballanced boat and at wot, boat should ride near paralell to water surface. If engine sits at best transom height with no back or over transom splashes whatoever, just near flat wake out engine, the only thing lefy would be to optimize engine rpm.

Were 4400 revs measured with a tach, if so, a seems very low. Check min-max wot rpm factory working parameters and select a prop that gives at least middle to near fully max rpm at wot. You definitely are lugging engine at 4400 need -less prop pitches so engine + revs happily.

Happy Boating
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The motor cant be trimmed out far enough. That is the problem. Yes, 4400 was measured with a tach. The recommended rpm is 4500 from what I've been told by other members in the evinrude motor section of this forum. I also just did a search and found this page on iboats http://boatspecs.iboats.com/Evinrude__18803__18hp__1968/bpe/57e10554 that lists 4500 as the suggested rpm
 
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james__12345

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Re: effect of trim on speed

82rude said:
james,i believe your just asking too much from that little motor on such a boat.whats the rating of that boat?i went on line and all the lonestars I found in that length had 40 to 55 hp. motors.a general rule of thumb is 70 percent min of stated hp rating.youre over 50 percent I believe,unless your lonestar is somehow dractically different then the ones I seen.
Click to expand...

I went and measured the boat again. It is actually 16 foot 4 inches long. It has four seats that run across it, and three of those seats have tanks under them for floatation (not foam, but tanks). The other seat is the livewell. At the top of the transom the boat is 48 inches wide. At the widest point the transom is 50 inches. The transom is 18 inches tall at the edges, the area where the motor mounts is 15.5 inches. At the widest point of the boat at the top it is 58 inches. At the widest point at water level (where the rib goes down the side of the boat) it is about 51 inches wide. You may have access to some information that I haven't been able to find, but if you can find a boat that matches those numbers so I could get the specs on it as far as weight and hp rating that would be amazing.
 
Sea Rider

Sea Rider

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Re: effect of trim on speed

So you can't trim engine to be perpendicular once boat is floating, why is that so. 4500 revs is the min or max rpm for than engine ? Low compared to other 18 HP brand engines which runs 5300-6000 at wot.

Happy Boating
 
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james__12345

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Re: effect of trim on speed

No the motor can not be trimmed to be perpendicular while running. As I clearly stated in my first post, the pin is in the last hole and can not be adjusted out any further. I don't know if the 4500 is a min or max. Everything I have saw lists it as the operating rpm not as part of a range, like it is in the link I posted. I can't find a range anywhere.
 
nwcove

nwcove

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Re: effect of trim on speed

james__12345 said:
I have a 16 foot lonestar mod v aluminum boat, with an 1968 18 horse evinrude on it. According to the formulas I've found for determining prop slip the theoretical top speed for this motor is a little over 24 mph. I'm only getting about 17 out of it. I also read that 10 percent slip is a decent expectation but mine is closer to 30 percent.
Click to expand...

with all do respect, the " perfect world" math rarely ever works out! but if its trimmed in a bit to much, you may be missing out on 1 or 2mph.
 
jimmbo

jimmbo

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Re: effect of trim on speed

Rpm range for that engine is 4000 - 5000. what pitch is the prop you have on it?
 
Frank Acampora

Frank Acampora

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Re: effect of trim on speed

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James: having the engine tucked too far in will drastically affect speed and performance. The engine will constantly be pushing the bow down, increasing wetted surface and slowing the boat.

As a POOR example, On this boat, I tuck the engine all the way in for the hole shot. The boat almost immediately goes to 45 MPH. Then, as I power trim out (away from the transom) The tachometer and speedometer both rise until a top speed of 60 is reached.

I don't know where, but "Wedges" are sold specifically to accommodate boats with transoms too tilted or not tilted enough. These fit under the transom clamps and allow you to get more or less engine tilt as needed.
 
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james__12345

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Re: effect of trim on speed

The prop is a 9x10. Frank, the numbers you gave are really the type of thing I was looking for to get some rough idea of how much of a difference it could really make. It sounds like it's worth trying to fix. I didn't realize anyone made the wedges commercially. I figured I'd have to make something my self. I'll have to look into those and see what I can find.
 
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james__12345

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Re: effect of trim on speed

All the wedges I can find are for bolt on motors. Mine is a clamp on. There are some bolts through the transom right beside the motor bracket, and one in the middle of the motor, but it doesn't look like those bolts would work for holding any of the wedges I can find in place. Looks like I might have to make them after all.
 
Fleetwin

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Re: effect of trim on speed

james__12345 said:
All the wedges I can find are for bolt on motors. Mine is a clamp on. There are some bolts through the transom right beside the motor bracket, and one in the middle of the motor, but it doesn't look like those bolts would work for holding any of the wedges I can find in place. Looks like I might have to make them after all.
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James,

Just for experimentation, you can buy wedge stock at Home Depot for about $2.00. The stuff you would use to frame in a door.

Once you get the right angle combination, you can easily re-create the angle with hardwood.

With all that said, I assume:

1. Your engine is a short shaft (15"). If it's a long shaft (20") you have a whole new issue.
 
J

james__12345

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Re: effect of trim on speed

I hadn't thought about using shims like that. I was thinking whatever I had would need to be bolted in to stay in place, but now that you mention that, I could pretty easily use some duct tape to hold them to the motor bracket just for a quick test before I cut a wide piece to span the bolt holes. As far as the shaft length, its about 17 so the cavitation plate runs about an inch and a half below the bottom of the boat. I know thats not really an ideal set up, but for now thats the best I can do.
 
Fleetwin

Fleetwin

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Re: effect of trim on speed

james__12345 said:
As far as the shaft length, its about 17 so the cavitation plate runs about an inch and a half below the bottom of the boat. I know thats not really an ideal set up, but for now thats the best I can do.
Click to expand...

That's a lot of your issue. You may want to look at ways to raise the engine. One easy, but not cheap, way to do it is with a CMC PT-35 Power Tilt and Trim. You can sometimes find good used ones on ebay-cheap. They can be set up to give you and inch or two lift plus you get power tilt and trim.:)

Be careful though. I'm not sure what lower unit you have on your eighteen. Some of the older ones had water intakes that were rather high on the leg.

Here's the CMC PT-35. ***You would also need the adapter for "clamp" style engines.

http://www.iboats.com/Electric-Hydr...9267279--session_id.799476057--view_id.392343
 
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james__12345

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Re: effect of trim on speed

I already rebuilt the transom a little earlier this year because it was rotten, so I know how to do it all. It would be much easier for me to just redo the transom and raise it than to pay over $500 for that. This boat has a notch down in the transom for the motor, so if I just set it up to go straight across it would solve the problem. The main intake is down by the prop. There are also intakes right above the cavitation plate. Once I get the trim fixed I may consider building it up if I'm still not happy with the speed i'm getting, but I'm going to wait till the trim is right before I do that.
 
J

james__12345

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Re: effect of trim on speed

Well, I finally got to test out the shim I made. Something strange happened though. When I finally got it set so that the motor was completely straight while running, at random the motor would rev up and the rpm's jump by about 400 with out any increase in speed (a small drop if anything). I assumed it was ventilating, and lowered the motor down one hole, which fixed the issue, but leaves the motor trimmed in a little still. How would a motor that is "too long" be venting with the trim set to what's supposed to be right, or is there something else I should be looking for here?
 
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