Engine too low in water?

[garretbishop]

Hello all. I recently got an 11 foot Saturn boat and a short shaft 6hp Yamaha. The transom on the boat is 14.5 inches but the transom length (or shaft length) on the outboard is 17.3. The engine was new so I was still breaking it in this past weekend and only did about half throttle to 2/3 throttle but it seemed like it was struggling compared to other videos of 6hp's on inflatables I saw. Do you think it would benefit me much to get the cavitation plate up to the bottom of the transom? I wanted to check before messing with the integrity of the transom on the boat. Thanks!

[The Gurnard]

Hi garrett .. two things come to mind.. lift the outboard as high as you can so the clamps still fit inside the transom plate.(use a block of wood on the transom to lift the OB to this height)

More important .. Wait till you run your OB in before trying full throttle. I run small boats on 6HP and they need full throttle to get on the plane..after that I can back of a bit.

They often need careful weight trimming to get the best out of them too.. Example .. I have to use a tiller extension to get my weight in the centre of the boat.

If I sit at the back..it bogs down and cant get speed up. They wont plane if I carry too much weight either . Example .like another passenger..but I have seen 3m+ SIBs plane with two small adults..so it might be possible.

Hope that helps some ..

[Pikey Dave]

Originally Posted by garretbishop

Hello all. I recently got an 11 foot Saturn boat and a short shaft 6hp Yamaha. The transom on the boat is 14.5 inches but the transom length (or shaft length) on the outboard is 17.3. The engine was new so I was still breaking it in this past weekend and only did about half throttle to 2/3 throttle but it seemed like it was struggling compared to other videos of 6hp's on inflatables I saw. Do you think it would benefit me much to get the cavitation plate up to the bottom of the transom? I wanted to check before messing with the integrity of the transom on the boat. Thanks!

I thought a short shaft engine was 15”[emoji848]

[The Gurnard]

They all vary PD..my Yamaha 6HP ..same as garrets is 17.3 inch..and its called a short shaft or as others call them.. standard shaft

[The Gurnard]

Yamaha web site says the sell 6HP 4 stroke at 15” or 20” shaft length
https://yamahaoutboards.com/en-us/ho...5-2-5-hp/6-2-5

Here is my Yamaha which I bought as the 15” short shaft (right of photo) its actually 17.3 from AV plate to clamp inside top. Left photo is my wee Mariner two stroke short shaft ..and its 17 “ from AV to inside clamp top. My 25HP Mariner short shaft is same.

So unless I have been shafted all these years ..when they say short shafts..which some also refer to a Standard shart ..is actually a bit longer..so I get more metal for my money. Does anyone actually have a 15” length from AV plate to inside top clamp..or am I measuring wrong places ?



Its a funny old world when 15 inches doesn’t mean its 15” ? Its possibly 15" from clamp centre..but I was always under the impression it was measured to inside top of clamp..and suspect that where garrett measured his from too..but he can confirm that

[garretbishop]

Originally Posted by Pikey Dave

I thought a short shaft engine was 15”[emoji848]

I thought so as well but the Yamaha Short ones are apparently 17.3. Wasn’t sure if a few inches affected it that much

[garretbishop]

Yes that’s exactly how I was measuring! I wasn’t sure if that was correct or if I had gotten the wrong one at first as I am completely new to this haha. I guess it means from where the clamps are. I’ll try putting it up higher on the transom. The way the transom is I can’t move it up much but I will take a video of it this weekend and see how it does full throttle open.

[Fenlander]

I was stressing about feeling inferior Donny but indeed my 17yr old Yamaha 2-stroke is as long as yours... 17.3 inches. I have the original handbook and that is exactly the spec... 17.3 inches.

Interesting how consistent these outboards are and how consistent the SIBs are in having over-short transoms.

And yes Garrett particularly on a minimal HP outboard for the SIB yes a few inches can make a difference. Both to performance and splash. But as recently discussed on another thread it's a personal thing how much effort you put in to get the match spot on.

[pagick]

I had a sib with a block sitting on the transom. Screwed in place from dealer. The clamps were near the top of original transom. In choppy water the engine fell into the boat once. After that I sandwiched the transom/ block with stainless sheets in and out with bolts squeezing them together. No problem after! Although I also never went out with out the bolts in as well. Do the 6 hp have mounting bolts?

[jeffstevens763@g]

standard shaft 15-17 inches i found that out on the excel had to lift it 40 mm wanted 50 mm but the clamps would have been off the plate

[beamishken]

Originally Posted by jeffstevens763@g

standard shaft 15-17 inches i found that out on the excel had to lift it 40 mm wanted 50 mm but the clamps would have been off the plate

I was good pals with a boat builder until he retired & he always said short shaft boat transom should be 15 -16" & long 20-21" why have boat builders started making 14" transoms?

[jeffstevens763@g]

Originally Posted by beamishken

I was good pals with a boat builder until he retired & he always said short shaft boat transom should be 15 -16" & long 20-21" why have boat builders started making 14" transoms?

i was told by a mechanic that the clamp should not rest directly on top of the transom and that the screw clamps and through bolts should do the work, personally i haven't followed that advice. i guess engine manufacturers giving legs of 17 inches allow for that plus lifting for better performance

[mm289]

Just to confirm thge above really, Have posted a load of pics on my thread about engine height om my Frib.

Transom is 15", both my Mariner engines are 17 - 17 1/2".

Problem is you can't lift the engine far because the clamps start to come in above the transom pad, especially on the smaller 6HP engine


Seems odd that they are all like this, wouldn't mind a 17" leg if the clamps were a lot deeper, i.e. the "U" shape of the mounting bracket was deeper.


Wonder if this varies between manufacturers


Cheers,


Paul.

[jeffstevens763@g]

Originally Posted by mm289

Just to confirm thge above really, Have posted a load of pics on my thread about engine height om my Frib.

Transom is 15", both my Mariner engines are 17 - 17 1/2".

Problem is you can't lift the engine far because the clamps start to come in above the transom pad, especially on the smaller 6HP engine


Seems odd that they are all like this, wouldn't mind a 17" leg if the clamps were a lot deeper, i.e. the "U" shape of the mounting bracket was deeper.


Wonder if this varies between manufacturers


Cheers,


Paul.

if the engine is staying on the boat you could bolt it on only the same as using the two clamp screws really

[mm289]

Unfortunately on mine it comes off quite frequently either because I fold the boat away or when I swap the 15HP with the 6HP, which I tend to do when I am just pootling about on the river.


Cheers,


Paul.

[Richd]

Interesting this talk on having to raise outboards. My 2stroke Mercury 5hp also measures 17.3", but the manual says that the anti-cav plate should be 2-2.5" below the bottom of the transom. Are our tubes effectively making the boat sit higher than a hard boat, is that why they work better level with bottom of transom?

Actually, no, that should help if so!

[The Gurnard]

Originally Posted by Richd

Interesting this talk on having to raise outboards. My 2stroke Mercury 5hp also measures 17.3", but the manual says that the anti-cav plate should be 2-2.5" below the bottom of the transom.

Hi Richd ..yup my 25HP Mariner manual also says up to 2” below transom..as do many older manuals

The reason for that is there are many types of boats..and to say the AV plate should be level with transom bottom for all boats..is not correct. These older two strokes were more often used on displacement type boats which benefited from the AV plate being below the transom by up to two inches as drag was not an issue with the slower speed and heavier boats

https://www.boats.com/how-to/the-out...t-adjustments/

In the link..you will see some boats even benefit with AV higher than transom bottom.

Only one way to get best boating experience ..and that is to experiment ... as every hull type is different. Alter one thing ..example AV depth and it affects another thing.. example ventilation.. so chose the best compromise for your style of boating and your particular boat. To say there is a hard and fast rule ..is absolutely loco

Hope that helps some.

[Fenlander]

Similar to The Gurnard I'd reckon the advice about the 5hp being 2" plus below the bottom of the transom is because it is more likely to be used on a displacement hull where you want to be sure of your grip on the water and a bit of drag isn't an issue.

My last few outboards (Mercury and Mariner 10/15hp, Yamaha 15hp, Suzuki 20hp, Mariner 9.9hp) all had the same advice of the AV plate level or up to 1" below hull/transom... I guess this consistent advice is in line with more lightweight/planing smallcraft about now compared with "the old days".

I'm glad that in my experience setting or lifting outboard saddle positions to bring the AV plate level (or thereabouts) with the hull hasn't produced any of the threatened doom laden events of ventilation and loss of prop grip.

[beamishken]

When I'm fitting an engine to a rib I usually start level or 1" up with the av plate . Unless its a displacement plodder you will get loads of drag and spray by setting the av plate below the hull.

[jeffstevens763@g]

Just out of intrest my transoms 15 inhes leg 17 1/2 inces plate 1/2 inch below keel.