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Motor mounted too low?

P

Peter Devil Ears

Regular Contributor
Hi. I believe my motor is mounted too low. When I fitted it, I thought it was correct, the bottom of the boat is level with the cavitation plate in my opinion. I have a high spray of water behind the boat, so bad that I cannot pull someone on skis. I checked previous threads and everyone states that it means the motor is mounted too low. My question is: do I really need to drill more holes in my transom and re-mount the motor higher, or will a hydrofoil help? The hydrofoils at my local dealer are expensive and need drilling in the cav plate to mount. Thanks for any assistance.
 
How about a picture of the cavitation plate lined up on the keel side view. "Bottom of boat is level with cavitation plate in my opinion"; means what? In my experience a motor mounted too high will cause your issue not too low. Also were is the motor pinned?
 
Here is a picture, I think this angle is best.
Motor position 2.jpg
 
You're close, but you look to be low. Easiest way to do it? Take a straight edge from the bottom of the boat to the cavitation plate. In my experience, you can be almost an inch high before the motor starts spraying water like you describe (I called it Rooster Tailing). One other thing to look at is the trim angle of the motor when you're running. That'll do exactly what you're describing as well. One thing to look at too, if you're running too high on the transom water will spray out from the sides of the prop too. If you're trimmed out too much it'll be more of a rearward direction. Save your money on those whale tails. While they will help the boat plane and maintain it's running attitude better, they'll also suck top end speed too.
 
Peter Devil Ears said:
Hi. I believe my motor is mounted too low. When I fitted it, I thought it was correct, the bottom of the boat is level with the cavitation plate in my opinion. I have a high spray of water behind the boat, so bad that I cannot pull someone on skis. I checked previous threads and everyone states that it means the motor is mounted too low. My question is: do I really need to drill more holes in my transom and re-mount the motor higher, or will a hydrofoil help? The hydrofoils at my local dealer are expensive and need drilling in the cav plate to mount. Thanks for any assistance.
Click to expand...

From your picture you are about 2 holes or 1 1/2 inch at least too low. Without the being trimmed out too far, or rooster tail as a result, you need to get the engine up.

Course you or a comrade could determine the cause of the problem by just looking down on the engine and spray patterns associated with it while doing what you don't like.

Mark
 
With the correct procedure used for drilling you would be able to raise the motor one hole without having to redill the transom.
 
Fish-a-Palooza said:
in a perfect world thats what should be in line with the bottom of the boat,
Click to expand...

Opinion!



History of 50+ years of boating including off shore is that it depends on a lot of things including hull shape and everything to do with the boat, engine mounting position, HP and prop and everything to do with the engine, usage aka offshore or large lakes, or water toys, or calm lakes, or rivers, water traffic, type and wakes, afternoon cruising, going as fast as possible, on and on.

Each case has it's sweet spot. No one boat can fulfill all requirements completely because of the above.

Mark
 
Here is a picture of the mounting, as you can see there are many holes from a previous motor, and lifting this motor by 2 inches will still require two extra holes for the bottom bolts.
Motor pinning.jpg

I always trim it all the way down, trimming it up at any amount will not eliminate the rooster tail. Here is a picture indicating the transom angle, the motor is unfortunately up now. The motor is a 175HP black max with a 19pitch prop.
Trim angle.jpg

The boat is used on large lakes to pull water toys. The other mounting holes in the transom were from a 115HP Suzuki that could not pull the water toys and was not very fast. It was also a short shaft motor that was not correct for this boat. I guess I made a mistake when I mounted this motor and need some advice before drilling more holes. Would lifting it up by 2 inches be the answer?
 
kimcrwbr1 said:
Measure 1/2 inch above the lowest point of the transom and then lower the cav plate level with the keel of the boat the cav plate should be close to the 1/2 to 1 inch above the plane of the hull. The holes can be patched.
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+1 but again it depends on the things I mentioned earlier, essentially everything. The main problem with it being too high would be ventilation in high sea state conditions or when pulling very heavy water toys where the prop does a lot of spinning but the boat doesn't move forward very fast.

You really have me confused with your comment about a 15" mid section on the previous 115 hp engine. You can't put a 15" engine on a 20" transom and have it work. The prop MUST see new water to function. Discounting racing boats, you need to be able to get in front of your boat and look down the hull to the engine's propeller and see the whole prop....just barely which the 1/2 to 1" above as mentioned will do.

I did notice your transom has a very slight slope and also that the tilt pin has been removed from your engine. That is good as you will need all the tuck in you can get in some instances of rough water or hard to move water toys with that slight angle of the transom.

Last: The holes I see in the transom do not appear to correspond to a 15" (short shaft) engine on that transom. That part really makes no sense to me but you are there and I am not. Looking at those holes and thinking of 5" of missing length you'd have just about half the prop directly behind the transom, no new water for it and "no way Jose" as the saying goes would that work.....unless you were running a 3 pt hydroplane at super sonic speeds which that doesn't seem to be the case.

Mark
 
What is that bracket at the lower left corner of this pic and is it below the plane of water from under the boat also the speed puckup could be further away from the motor also the bulb only needs to be slightly below the transom.
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That bracket is the speed sensor, the bulb on the right is the pickup for the fish finder. Neither of them could be problematic, they were there before and at that time (with the 1115HP) there was no rooster tail.

You really have me confused with your comment about a 15" mid section on the previous 115 hp engine. You can't put a 15" engine on a 20" transom and have it work.
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That is one of the reasons for removing that motor and selling it. The black max has the correct length. The 115HP Suzuki was not mounted directly to this transom, that is why the other holes may make no sense. The 115 was mounted on a large bracket type setup that pushes it back by 300mm and allows you to raise it (don't know what that bracket is called), since it was too short to fit correctly. This was the previous owners' answer to using that motor.
 
That previous item that was on the other motor is a spacing device. Moving the drive rearward gives the prop cleaner water, so it may have improved behavior with the prior motor. Some of them also include a raise and lower function and are a great solution to your current dilemma. They are called jack plates. They're kinda pricey though. Folks with multiple outboards and go fast outboards use them to fine tune their planing attitude.
 
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Texasmark said:
Opinion!



History of 50+ years of boating including off shore is that it depends on a lot of things including hull shape and everything to do with the boat, engine mounting position, HP and prop and everything to do with the engine, usage aka offshore or large lakes, or water toys, or calm lakes, or rivers, water traffic, type and wakes, afternoon cruising, going as fast as possible, on and on.

Each case has it's sweet spot. No one boat can fulfill all requirements completely because of the above.

Mark
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Of course it's an opinion. What do you think your post is? I said the same thing you did; I just didn't feel the need to post a resume or state every possible variable. I said, in a perfect world the cavitation plate is in line with the bottom of the keel. Any motor manufacturer will tell you to start there. Regardless; an inch or two low should not require physically moving the motor again. You should be able to adjust/trim for that.
 
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If you had a 15" Suzuki on a jack plate then that would make sense as the farther you set the engine back the shorter the midsection can be as the prop IS mounted higher than a straight line extension of the hull. Reason being is that boats run bow up normally and the farther back, the deeper the prop runs in the water. Running the same length midsection that corresponds to the height of the transom would require the plate to be jacked up as NHGuy mentioned.

Mark
 
Thanks for all the assistance, I will take the boat out one more time and make 100% sure that trimming it cannot sort the issue. If not I will lift it. Will let you know how it goes.
 
Feedback: no amount of trimming could get rid of the rooster tail. I then lifted the motor and pinned it onto the bottom holes. This lifted it by two inches. It's now running fine, no rooster tail! Thanks for all the assistance.
 
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