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Fishing Boards - Boats & Motors

Outboard motor height set up

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Crashnrondo

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7/22/12 9:04 AM CST

I have a Mirrocraft 1615 troller with a Merc 40hp 4 stroke and from day 1 it just didn't seem right. Max GPS speed was 26 and it would only stay on plain at WOT. I put a tach on it and found out I was only getting 5100 RPM's so I changed props and it helped, I gained 2MPH and 250 RPM but I still didn't feel the boat motor combination was performing as it should. With some research I figuired out my motor was mounted way to low or "buried" as some call it, it was mounted as low as it could go so I moved it up to the middle holes and gained another 2 MPH and I can finally cruise at 2/3 throttle and maintain 25mph. So looking at some pictures and motor set up threads my motor is still too low, the cavitation plate should be visiable while trimmed out and at cruising speed so last night I moved it up another hole. I gained another 1/2 MPH or so but it looks like I can come up another hole looking at the cavitation plate but this will put the motor in the bottom hole/top height. Is there any concerns with mounting an outboard up that high on an aluminum boat?

FYI-The boat still won't break lose in tight turns and the steering has become very neutral and my fuel consumption has been cut in half.

Thanks, Ron

centralwiwalleyeslayer2

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8/21/12 4:24 PM CST

i got a 30 4 stroke on my 14 footer had problems planing out with the prop the dealership put on with a load droped down from a 12.5 to a 11 pitch didnt lose any speed 28.5 by myself and 26 with 2 big guys and a 90 pound lab she jumps out of the water...

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ibfishn

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8/9/12 10:51 AM CST

Crashnrondo, as a general rule of thumb a good starting point is to have the motor mounted with the cavitation plate level with to an inch above the bottome of the hull. If you are having a porpoising problem you either have too much weight in the back of the boat or not enough HP. Sounds like you need to drop the motor back down, most times you won't have neutral steering until you get up on plane and trim the boat out. If your steering is neutral at WOT with no trim then your motor is already too high.

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eyefish

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8/6/12 1:23 PM CST

Thank you I appreciate your response.

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Mr. Fish 2

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8/6/12 4:14 AM CST

eyefish

You will need a short shaft for that boat.

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eyefish

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8/5/12 9:28 PM CST

Hey guys, hope I am on the right forum. I have a 9.9 long shaft. And was thinking about buying a 14 ft flat bottom rated for a 20 hp max. The transom is 15 inches, will the. 9.9 be too much or will it work?

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Crashnrondo

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7/29/12 9:35 AM CST

luremaker, I'm in Neenah and the boat is maxed out at 40hp. They usual package the boat with a 25hp and that has to be a dog. I do have power trim and tilt but after the move to the highest motor setting I can't trim the motor without it porpoising and it looks like it should still move higher. I'm considering moving it back down 1 hole just to get some trim movement back and then just play with different props.

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luremaker

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7/28/12 11:24 AM CST

where are you located?

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luremaker

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7/28/12 11:22 AM CST

I know in allot of cases when buying a boat, money is a factor. Best thing to do right from the get go is to max out the horse power for the boat, knida eliminates allot of grief. I would hope this motor has trim and tilt on it. If not you may have to play with the trim setting. Getting neutral torque on the wheel means your motor is happy at that trim setting. Engine height and props play the most in how the boat will perform. A fully loaded boat should run close to max rpm's. Sounds like the motor is lifted as high as it will go and you may be close with the prop. Buy a manual jack plate if you are not blowing out on turns. Remember that tilting your motor while running will help lift the boat causing less drag= higher speeds also. You almost need someone that has played with these issues to help you out, may be your best bet. 16' boat with a 40hp motor? sounds a little under powered.

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Crashnrondo

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7/23/12 7:53 AM CST

jal24, they do make the motor in both short and long shaft but my boat is only made with a 20" transom.

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jal24

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7/22/12 8:15 PM CST

Do those 40 4 strokes come in two different shaft lengths?

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Crashnrondo

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7/22/12 5:57 PM CST

Sslayer, the picture that shows it too low is exactly how mine looks right now and that is after moving it up 2.5" already. I'm going to move it up to the highest hole and test it Monday. My boat came rigged from the dealer and I'm not sure why, kinda funny I ran into a guy on the river today with the same boat/motor from another dealer and his was mounted as low as possible also. I took him out and he was floored at how much better it handles and the additional speed mine has, he said he would be moving his up tonight.

The reason I asked about the height is I thought I saw something about the effects of the weight being higher on the transom doing bad things. I do use a transom saver for trailering...

Thanks guys

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sslayer

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7/22/12 10:22 AM CST

Moving up to the top mounting hole is okay provided the water intakes still have suction and you don't have handling issues.

Perhaps whoever mounted the motor originally set it to low from the start. If the Dealer did it ask why!

Here are a couple of pictures of a motor being too low and just right.

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Steve @ G & S

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7/22/12 9:14 AM CST

I don't believe that moving the motor up to the top hole will have any harmful effect on your boat at all. Go ahead and do it, just remember to pay attention to your telltale stream when you're running up on plane. You don't want to raise the motor so high that it starts to suck air in the water intakes. That's a sure recipe for a new powerhead!

Formerly Steve @ G & S

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