Engine too low?

[Dkillough]

I recently purchased a 91 f470 with a 2012 Merc 40efi 20" shaft. When running at speed, even on plane, an enormous amount of water shoots up between the transom and the outboard. I've read that this is generally due to the motor being mounted too low. After some reading and measuring, I suspect this is the case. The older F470s had a lower transom than current models, and a shorter shaft would probably have been better. I'll link to some video and pictures below, just hoping to have confirmation before I start either a) building the current transom up a little b) attempting to replace the transom all together or c) sell this motor and source a new one.

https://photos.app.goo.gl/CjkXR3zVJmpK6ff47

[Fenlander]

It would be helpful to know the current transom measurement.

It does seem getting going you are not helped by fuel tank and battery etc right next to the transom.

[Dkillough]

Originally Posted by Fenlander

It would be helpful to know the current transom measurement.

It does seem getting going you are not helped by fuel tank and battery etc right next to the transom.

Yeah, moving some weight forward would probably help keep the bow down a little, but the problem seems more related to the way the outboard is moving through the water. There's still plenty of freeboard, the transom isn't submerging.

I taped it, the transom is 15" from top to bottom in the center. The motor is already mounted a little higher than that in its own bracket. I'll add a couple more pictures to the album momentarily.

[Steve509926]

A 15" transom with a 20" shaft would suggest its a "little" low in the water! The AV plate should be level +/- 1" with the bottom of the transom/keel.
Easiest solution would be a different OB. Cheapest solution is to extend the transom with two stainless steel plates and a HDPE core.
Most difficult IMO, would be to fit a new transom.

Agree with Fenlander, fuel tank and battery need to be moved forward. Maybe it won't help with this problem, but petrol & water and electricity & water don't make good bed fellows.

[Fenlander]

If the SIB is a keeper I'd swap the outboard for a short shaft. Raising the transom that far risks strength issues, a higher C of G and looks odd.

[Steve509926]

Originally Posted by Fenlander

If the SIB is a keeper I'd swap the outboard for a short shaft. Raising the transom that far risks strength issues, a higher C of G and looks odd.

Me too

[jeffstevens763@g]

I’d agree with fenlander and Steve but for all the sibs I’ve owned I never came across a boat that I have now that is standard shaft transom but when I set my engine up to have the cav plate level with the keel it cavitated result is mine is two inches below now and runs well. OMO I would play with a transom extension to make sure five inch shorter leg works before I changed the engine.

[Steve509926]

Think we've been here before Jeff re: excels sitting high in the water.
My av plate is a good 25mm below the keel and at times I think it could be a bit lower, but a 125mm extension is 33% increase on the existing transom height. However a transom extension is the cheapest option, and would confirm that at the existing transom height the prop is way too deep in the water. It's got to be worth a try

[Locozodiac]

Was the general load evenly distributed on deck while the motor was trimed to ride perpendicular with the AV plate paralell to the water level. There are far better ideas than to modify any transom height. To convert a 15 shaft transom to be powered wita a 20 shaft motor can fabricate an SS 4-5 inch transom mount riser and install it right at middle of the current transom.

Happy Boating

[jeffstevens763@g]

Originally Posted by Steve509926

Think we've been here before Jeff re: excels sitting high in the water.
My av plate is a good 25mm below the keel and at times I think it could be a bit lower, but a 125mm extension is 33% increase on the existing transom height. However a transom extension is the cheapest option, and would confirm that at the existing transom height the prop is way too deep in the water. It's got to be worth a try

I’m just pointing out that before you spend go down the cheap test route I have no knowledge of the f470 I would guess a new engine but he might get away with a transom mod for little money

[Steve509926]

Originally Posted by jeffstevens763@g

I’m just pointing out that before you spend go down the cheap test route I have no knowledge of the f470 I would guess a new engine but he might get away with a transom mod for little money

👍 Agree.

Apparantley the f470 was built with 3???? transom heights. 15", 18" & 21" can't find out why? T'internet doesn't offer up any clues, apart from the 15" was for "beach landing quietly" - whatever that means.

Did a quick search for a 15" short shaft (ultra short shaft) and came up with absoultly nothing.

[spartacus]

Originally Posted by Steve509926

👍 Agree.

Apparantley the f470 was built with 3???? transom heights. 15", 18" & 21" can't find out why? T'internet doesn't offer up any clues, apart from the 15" was for "beach landing quietly" - whatever that means.

The F470 is a MilPro specification boat. On a standard shaft outboard, probably manual or gas lift assist, then beach landing with commandos or marines can be done quickly and covertly. That would be my take on it.

[beamishken]

Did a quick search for a 15" short shaft (ultra short shaft) and came up with absoultly nothing.[/QUOTE]

No such thing as ultra short shaft , a 15" transom is a standard shaft (often refered to as short shaft), 20" is long shaft 25 extra long.
The 18" shaft is an oddball & I thought was a special build for the rnli & only a few engines available with 18" shaft usually mariner models.
Often a couple of inches variation in normal leg lengths depending on engine manufacturer so some engines will run deep when dropped on a standard length transom & some not so deep

Sent from my SM-G950F using RIB Net mobile app

[Steve509926]

Originally Posted by beamishken

Did a quick search for a 15" short shaft (ultra short shaft) and came up with absoultly nothing.

No such thing as ultra short shaft , a 15" transom is a standard shaft (often refered to as short shaft), 20" is long shaft 25 extra long.
The 18" shaft is an oddball & I thought was a special build for the rnli & only a few engines available with 18" shaft usually mariner models.
Often a couple of inches variation in normal leg lengths depending on engine manufacturer so some engines will run deep when dropped on a standard length transom & some not so deep

Sent from my SM-G950F using RIB Net mobile app[/QUOTE]

I stand corrected.

But I did read it somewhere, can't find the F470 Web page where I read "ultra short shaft" but they also mentioned ultra long shaft - 30".

[jeffstevens763@g]

I think I seen somewhere that there is a variation on the short shaft 15-16.5 inches on some makes of engine.

[beamishken]

Originally Posted by jeffstevens763@g

I think I seen somewhere that there is a variation on the short shaft 15-16.5 inches on some makes of engine.

Definitely a fairly large difference in lengths of shaft between models from different makers
Iirc a 25 or maybe a 15 2 stroke yam is 16.5" short & 21.5"long. Some makers must have rubber tape measures[emoji6]

Originally Posted by Steve509926

No such thing as ultra short shaft , a 15" transom is a standard shaft (often refered to as short shaft), 20" is long shaft 25 extra long.
The 18" shaft is an oddball & I thought was a special build for the rnli & only a few engines available with 18" shaft usually mariner models.
Often a couple of inches variation in normal leg lengths depending on engine manufacturer so some engines will run deep when dropped on a standard length transom & some not so deep

Sent from my SM-G950F using RIB Net mobile app

I stand corrected.

But I did read it somewhere, can't find the F470 Web page where I read "ultra short shaft" but they also mentioned ultra long shaft - 30".[/QUOTE]Yeh you do get an ultra long 30" shaft but they are usually in the large hp engines

Sent from my SM-G950F using RIB Net mobile app

[GARYSHORT]

Short shaft. Your transom is not designed for a long shaft and you will continue to have problems even with all of the mods discussed here.