Engine height on transom. Did I go to far?

AEROCOOK

AEROCOOK

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Jul 28, 2004
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In my quest for more speed I have raised my motor up one hole setting on the transom but now the boat porpoises unless it is trimmed all the way in and I put additional weight in the bow.
Here are the details on the boat...
1987 16.5' aluminum bowrider with a 1988 110hp Johnson.
Heres the deal, I also removed the aftermarket cavitation plate fins at the same time so Im not sure which change contributed most to the excessive porpoising that the boat now has.
Idealy I do not want to put the fins back on as they made steering more difficult and when I took them off I welded the holes back up.
So my primary question is whether or not simply raising the motor on the transom could cause the condition?
 
Bondo

Bondo

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Re: Engine height on transom. Did I go to far?

So my primary question is whether or not simply raising the motor on the transom could cause the condition?
Click to expand...

Ayuh,... Entirely possible.... You might need Setback...
 
Mark42

Mark42

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Re: Engine height on transom. Did I go to far?

I would prefer to have a slightly slower and well handling boat than one that is fast and not as well behaved.

Put the motor back down and look for weighs to lighten the load or get a bigger motor.
 
AEROCOOK

AEROCOOK

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Re: Engine height on transom. Did I go to far?

Mark42 said:
I would prefer to have a slightly slower and well handling boat than one that is fast and not as well behaved.

Put the motor back down and look for weighs to lighten the load or get a bigger motor.
Click to expand...

Thanks for the reply.
I will put the motor back down one hole (where it was originally) and see what happens.
A bigger motor isnt an option as Im already 20hp over the power rating for the boat.
Im able to hit 40 mph with 3 people in the boat and the hole shot is exceptional to say the least so Im pleased with the performance.
My personal fault is that I have a "racers mentality", if theres a way to get even the slightest improvement I have to try... (I need to learn to leave well enough alone).
I will post the results in a few weeks when I go back up to the lake.
 
allpoints360

allpoints360

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Re: Engine height on transom. Did I go to far?

Where is the cavitation plate in relation to the bottom of the hull? At 40mph, I think it should be even with or no more than 1 inch below the bottom of the hull for a few reasons.

I would think the porpoising is related to removing the fin because it evens out when trimmed all the way in.

Sometimes, trim tabs can help with this.
 
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crb478

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Re: Engine height on transom. Did I go to far?

It would have been easier to put the fins back on, but since they are already welded up you can either drop it back down a hole or try some trim tabs. If you are not having any cavatation issues I would lean toward a pair of smart tabs. Try to do one change at a time so you know what caused the new condition.
 
4JawChuck

4JawChuck

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Re: Engine height on transom. Did I go to far?

I would install a jackplate and then you have infinite adjustability and more leverage on the bow from the setback. For your application a 6" manual jack is more than enough, I have a TH Marine 6" jackplate on my 16' fishin boat with 90HP Merc. Being able to adjust the motor height in small increments will enable you to fine tune the performance not to mention putting the motor back 6" will enable you to raise the motor even higher since the prop will be in the "bubble" of the transom wave that rises behind your boat at speed.

Installation of the jackplate and fine tuning the prop height gained me 5mph over a stock setup, raising the motor another hole would just cavitate the prop so I needed the adjustability. Just remember to do your testing fully loaded and empty to find the happy medium position, having the right prop height fully loaded is different than the no load position.
 
Bifflefan

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Re: Engine height on transom. Did I go to far?

AEROCOOK said:
I also removed the aftermarket cavitation plate fins at the same time so Im not sure which change contributed most
Click to expand...

Always make one change at a time. Or you end up like this, with a problem, not knowing what caused it or which one to put back to fix it.
 
AEROCOOK

AEROCOOK

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Re: Engine height on transom. Did I go to far?

bifflefan said:
Always make one change at a time. Or you end up like this, with a problem, not knowing what caused it or which one to put back to fix it.
Click to expand...
I knew someone was going to give me this good advice... I should have known better :)
 
AEROCOOK

AEROCOOK

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Re: Engine height on transom. Did I go to far?

4JawChuck said:
I would install a jackplate and then you have infinite adjustability and more leverage on the bow from the setback. For your application a 6" manual jack is more than enough, I have a TH Marine 6" jackplate on my 16' fishin boat with 90HP Merc. Being able to adjust the motor height in small increments will enable you to fine tune the performance not to mention putting the motor back 6" will enable you to raise the motor even higher since the prop will be in the "bubble" of the transom wave that rises behind your boat at speed.

Installation of the jackplate and fine tuning the prop height gained me 5mph over a stock setup, raising the motor another hole would just cavitate the prop so I needed the adjustability. Just remember to do your testing fully loaded and empty to find the happy medium position, having the right prop height fully loaded is different than the no load position.
Click to expand...
I would love to get jackplate, in fact I have been watching the classifieds for one but I just dont have much of a budget right now.
For now I just need to remember that I dont have a high performance boat and just find a decent set up and leave it alone.
 
AEROCOOK

AEROCOOK

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Re: Engine height on transom. Did I go to far?

allpoints360 said:
Where is the cavitation plate in relation to the bottom of the hull?

The cavitation plate is at least an inch above the bottom of the hull.
I only raised it because I read somewhere (on this forum) that to get the most performance you keep raising the motor till you get cavitation then go back down 1 hole. Apparently cavitation isnt the only undesired effect that can result from raising the motor.
BTW I have a brand new set of smart tabs (left overs from a previous project that I never finished) but I just hate drilling holes in my transom unless I absolutely have to,
Click to expand...
 
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swordfish25

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Re: Engine height on transom. Did I go to far?

Trim tabs wiil cure the promlem, and give you some more control of the lean, or weight .
 
allpoints360

allpoints360

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Re: Engine height on transom. Did I go to far?

The cavitation plate is at least an inch above the bottom of the hull.
I only raised it because I read somewhere (on this forum) that to get the most performance you keep raising the motor till you get cavitation then go back down 1 hole. Apparently cavitation isnt the only undesired effect that can result from raising the motor.
Click to expand...

Raising your motor can increase your speed, but can lead to overheating. I'd lower the motor first to see if this corrects the porpoising. If not, those smart tabs should do the trick.
 
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