Engine height

ChuckB · Sep 10, 2024

I bought my first boat last October. I did not notice at the time but there is flex in the transom so I think I should replace the wood but the engine height also look a lower than ideal.

The bottom bolt holes for the engine do not go through the wood part of the transom.

I would have to drill a new bottom holesto raise the engine but that would allow it be put in the wood section of the transom.
I was thinking I could raise it two hole locations at the top which would be about an 1 1/2 inches.
It does not look like it would be that bad to replace the wood in the transom. The corner cap un screw revealing the wood. I probably want to get a cherry picker for the engine.
Its a 1990 Lund Rebel so may not be worth the effort but it has a newer 4-stroke 30HP Tohatsu with power tilt plus a newer Minn-Kota Terrova and a Garmin 73sv fish finder.
Does this seem reasonable to tackle. Its probably not going to fail any time soon but it bugs me.

LDUBS · Sep 11, 2024

Sounds like you have a nice rig to me.

To answer your question, to me it does seem reasonable. Especially since you need to replace the transom wood anyway. The rule of thumb is for the cav plate to be even with the bottom of the hull, so I agree you have room to move it up.

I would also not like those lower mounting bolts not having a more robust backing plate. If by raising the motor to the right height means the lower bolts go thru the transom wood, then you have an added bonus. If not, I would be adding something much more extensive than those washers.

You will need to patch the existing lower bolt holes. If you have questions about doing that, I should leave it to those with more knowledge and experience than me to respond.

FuzzyGrub · Sep 11, 2024

It looks like the motor is all the way down on the transom. If so, the upper hole were drilled one increment too low. If the lower hole is drilled to the top of the slot, would in go thru the wood?

The best height depends on the boat, motor, the load, and how you plan to use it. One inch above, is generally best for top speed. Just have to make sure the water pick-up is working well. You also may get cavitation in tight turns.

As LDUBS said, even is usually the best overall position. Depending on boat, you may still get cavitation in tight turns.

One inch below, is where many mfg recommend to mount. Both of my Nissan/Tohatsus were recommended as such. For most mfg, that is probably just a “safe” recommendation. Look up your engine manual and check. From 1” below to 1” above, usually talking about 2-3mph difference in top speed.

I’d look to be able to adjust within this range and select the appropriate hole location. Testing is the only true way to find out which your prefers.

airshot · Sep 11, 2024

I would take a long straight edge pushed up to the bottom of the hull and let it stick out to the cavitation plate. We can't tell how parallel the cav plate is to the hull. When both hull and cav plate are parallel, then measure how far above or below it is. I am surprised you have wood in your transom, thought Kinds were better built than that. My 1992 Sylvan has an all aluminum transom, thought mist all good aluminum boats went to all aluminum transom by then. I would definitely check that wood and replace if any soft spots are found !

Ray Clark · Sep 11, 2024

Reasonable? Yes. There are a number of different ways to diagnose and tackle that transom. Search on transom work and read the threads to your heart's content. If it's in good shape, your Lund could give many years of good service.

I have a 1979 Lund S14 and it's a great hull. Absolutely dry, no leaks. I have enough confidence in that hull that I've had no issue doing a bunch of work and upgrades: new outboard, new trolling motor, upgraded electronics, new decks, trailer work, and so on.

ChuckB · Sep 11, 2024

FuzzyGrub said:
It looks like the motor is all the way down on the transom. If so, the upper hole were drilled one increment too low. If the lower hole is drilled to the top of the slot, would in go thru the wood?

The best height depends on the boat, motor, the load, and how you plan to use it. One inch above, is generally best for top speed. Just have to make sure the water pick-up is working well. You also may get cavitation in tight turns.

As LDUBS said, even is usually the best overall position. Depending on boat, you may still get cavitation in tight turns.

One inch below, is where many mfg recommend to mount. Both of my Nissan/Tohatsus were recommended as such. For most mfg, that is probably just a “safe” recommendation. Look up your engine manual and check. From 1” below to 1” above, usually talking about 2-3mph difference in top speed.

I’d look to be able to adjust within this range and select the appropriate hole location. Testing is the only true way to find out whi

airshot said:
I would take a long straight edge pushed up to the bottom of the hull and let it stick out to the cavitation plate. We can't tell how parallel the cav plate is to the hull. When both hull and cav plate are parallel, then measure how far above or below it is. I am surprised you have wood in your transom, thought Kinds were better built than that. My 1992 Sylvan has an all aluminum transom, thought mist all good aluminum boats went to all aluminum transom by then. I would definitely check that wood and replace if any soft spots are found !

ChuckB · Sep 11, 2024

Here is the straight edge. The main reason I want to raise it is to be able to get the bottom bolt in the wood part of the transom.
At the very top of the bottom elongated hole it would be very close right now. My plate is 1 3/4 inch below the bottom of the boat so maybe raise it one hole on the top. The engine manual says from 0.2 - 1 inch below the bottom of the boat. That would put it 1 inch and should provide enough clearance from the bottom of the wood reinforcement.

LDUBS · Sep 11, 2024

FuzzyGrub said:
It looks like the motor is all the way down on the transom. If so, the upper hole were drilled one increment too low. If the lower hole is drilled to the top of the slot, would in go thru the wood?

The best height depends on the boat, motor, the load, and how you plan to use it. One inch above, is generally best for top speed. Just have to make sure the water pick-up is working well. You also may get cavitation in tight turns.

As LDUBS said, even is usually the best overall position. Depending on boat, you may still get cavitation in tight turns.

One inch below, is where many mfg recommend to mount. Both of my Nissan/Tohatsus were recommended as such. For most mfg, that is probably just a “safe” recommendation. Look up your engine manual and check. From 1” below to 1” above, usually talking about 2-3mph difference in top speed.

I’d look to be able to adjust within this range and select the appropriate hole location. Testing is the only true way to find out which your prefers.

"I’d look to be able to adjust within this range and select the appropriate hole location. Testing is the only true way to find out which your prefers."

This is such a good idea!

thill · Sep 12, 2024

I went through this with one of my previous tin boats. It ran well, but I was trying to make it "right" with the cavitation plate @ the hull bottom. So I did what it took to get the engine up, and it gained 2 mph, but it cavitated on anything but a straight run in flat water.

I ended up putting it back where it was, and it again ran well at all speeds and in turns.

So my question to you is, WHY are you doing it? To gain speed? To keep the motor from dragging or being hard to steer? Or just because it's "right" according to sources?

If the boat runs and handles well, I would replace the wood and leave it alone. I might add a block inside, across the bottom bolts, probably out of Azek or a similar material. Remember, the engine PUSHES on the bottom bolts, so much less stress is on them, and that's why you don't see any deformation in the metal. In other words, it's fine as far as strength.

If you are unhappy with the performance, then drill new holes, replace the wood, patch the aluminum transom and so on, and hope that all that work and extra holes in your boat are worth it. It looks like there is a panel on the transom where the motor sits, so the easy fix is to replace that panel and make sure it is sealed well.

I hope it works out and that you are happy with the final results.

MrGiggles · Sep 12, 2024

As mentioned, someone drilled the holes wrong. The bottom hole should be near the top of the slot. With how it is setup now, you have no adjustment.

Unfortunately, short of a welder there is not easy way to fill those holes and redrill them.

ChuckB · Sep 12, 2024

MrGiggles said:
As mentioned, someone drilled the holes wrong. The bottom hole should be near the top of the slot. With how it is setup now, you have no adjustment.

Unfortunately, short of a welder there is not easy way to fill those holes and redrill them.

True, but the wood part of the transom is around 10 maybe 10 1/2 inches from the top. The top of the sloted hole is 10 inches down

LDUBS said:
Sounds like you have a nice rig to me.

To answer your question, to me it does seem reasonable. Especially since you need to replace the transom wood anyway. The rule of thumb is for the cav plate to be even with the bottom of the hull, so I agree you have room to move it up.

I would also not like those lower mounting bolts not having a more robust backing plate. If by raising the motor to the right height means the lower bolts go thru the transom wood, then you have an added bonus. If not, I would be adding something much more extensive than those washers.

You will need to patch the existing lower bolt holes. If you have questions about doing that, I should leave it to those with more knowledge and experience than me to respond.

So a lower mount transom support plate would probably fit. I have s support plate on the top bolts. I guess pushing forces are not a problem because the whole bracket set against the transom but for pulling having the plate would be helpful.

LDUBS · Sep 12, 2024

ChuckB said:
So a lower mount transom support plate would probably fit. I have s support plate on the top bolts. I guess pushing forces are not a problem because the whole bracket set against the transom but for pulling having the plate would be helpful.

I think by lower mount support plate you mean a backer plate for the lower mounting bolts. Yes, I would add something for sure. That would be in addition to replacing the bad transom wood.

I will add I think @thill makes a very good point in post #9. Like he says, if you are happy with performance the way the motor currently sits, then save yourself some hassle of having to patch the old mounting holes and leave it as is. All that would really be needed after the transom wood is replaced is to add a backer inside for the existing lower mounting bolts. You could even use a short length of hardwood or exterior ply scrap as a backer. If you ever decide you want to adjust the motor height, then you haven't really lost anything.

ChuckB · Sep 13, 2024

LDUBS said:
I think by lower mount support plate you mean a backer plate for the lower mounting bolts. Yes, I would add something for sure. That would be in addition to replacing the bad transom wood.

I will add I think @thill makes a very good point in post #9. Like he says, if you are happy with performance the way the motor currently sits, then save yourself some hassle of having to patch the old mounting holes and leave it as is. All that would really be needed after the transom wood is replaced is to add a backer inside for the existing lower mounting bolts. You could even use a short length of hardwood or exterior ply scrap as a backer. If you ever decide you want to adjust the motor height, then you haven't really lost anything.

Thanks, might just use a peice of the plywood that I will need for the transom.

DaleH · Sep 13, 2024

No need for a cherry picker, I've lifted many an OB up to V4 140hp in size motors by hanging a come-along or chainfall rig off a tree branch or joist in a garage. Of course, most of those allowed for the proper accessory lifting ring that bolts to the flywheel.

One rainy day that I wanted to replace the rusty/carbon steel tilt tube on an OMC 90hp, we rolled into one of those self-serve car wash bays and hung the come-along off the heavy steel I-Beams above. Got it replaced with a SS one pronto! Had the whole place to ourselves to boot ... lol!

thill · Sep 13, 2024

MrGiggles said:
As mentioned, someone drilled the holes wrong. The bottom hole should be near the top of the slot. With how it is setup now, you have no adjustment.

Unfortunately, short of a welder there is not easy way to fill those holes and redrill them.

Maybe I'm wrong, but it looks like there is an aluminum panel between the engine and transom skin.

If he wants to proceed, he could simply replace that outside panel, sealing the perimeter and the old holes well. Then, drill new ones as desifed. Fill the inside of the holes with sealant, and there you go.

That would be the easiest and cleanest way to close those holes that I can think of.

LDUBS · Sep 13, 2024

thill said:
Maybe I'm wrong, but it looks like there is an aluminum panel between the engine and transom skin.

If he wants to proceed, he could simply replace that outside panel, sealing the perimeter and the old holes well. Then, drill new ones as desifed. Fill the inside of the holes with sealant, and there you go.

That would be the easiest and cleanest way to close those holes that I can think of.

Huh, seems like that would work. Like an oversized aluminum transom pad. I think I would butter the whole thing with whatever the preferred goop might be.

ChuckB · Sep 15, 2024

DaleH said:
No need for a cherry picker, I've lifted many an OB up to V4 140hp in size motors by hanging a come-along or chainfall rig off a tree branch or joist in a garage. Of course, most of those allowed for the proper accessory lifting ring that bolts to the flywheel.

One rainy day that I wanted to replace the rusty/carbon steel tilt tube on an OMC 90hp, we rolled into one of those self-serve car wash bays and hung the come-along off the heavy steel I-Beams above. Got it replaced with a SS one pronto! Had the whole place to ourselves to boot ... lol!

I have considered a chain hoist but then I need to mount it somewhere. I have heard that harbor freight will have the 1 ton cherry picker in the next parking lot sale. It would give me the option of pulling engine in different spots. The biggest pain with a cherry picker is storing it when not using it.

Crazyboat · Sep 15, 2024

Could you not just put a small hydraulic jack under the skeg and raise her that way? Taking caution that the engine doesn't fall over of course.

airshot · Sep 15, 2024

I use a HF chain fall in my garage. Wrapped a chain around one if my re enforced trusses and just leave it hang there for whenever I need it. I installed a hook off to one side to hang the excess chain. Pull out one car and use that bay for lifting work.

DaleH · Sep 16, 2024

Crazyboat said:
Could you not just put a small hydraulic jack under the skeg and raise her that way? Taking caution that the engine doesn't fall over of course.

That's what I do to even "fine tune" OBs to V6 and 500-pounds, but it ONLY works when the lower bolts - that are in slots on the bracket - have ample room up/down to move the top bolts up or down 1 hole or so.

The OP wants to mount his engine higher and he's out of room on the slots on his lower bolts as is.

Engine height

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