Bigger Trim Tabs?

[Tomsk]

Some time ago - before I purchased her, Andiamo had an engine swap from 5.7 litre petrol to 6.2 litre V8 diesel..

In their wisdom the owners at the time didn't consider gearing/revs/HP in their planning. Many moons later along comes I to purchase my first decent boat and buys her from Nationwide Boat Sales (dry marina) so have to buy on spec with just a service and survey. I loved the boat and wanted diesel.

The long and short of it is she doesn't quite get on the plane. Current top speed (measured this weekend in absolute flat seas) is 13.5knts with approx 30 stone of men perched on the bow and trim tabs down to max. Without men on the bow speed was 12knts.

Now don't get wrong - I would quite be prepared to stick to 12knts but I am so close to getting up there... what can I do?

I have changed the prop from a 16x16 to a 16x14.. this helped somewhat but I think it has taken the top speed down too far..

So I was thinking about the trim tabs.. Will larger trim tabs make much difference?

Advice and deliberation welcomed...

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Divemaster1]

The V8 diesel is probably a fair bit heavier then the petrol which may cause some of your "challenges".

What configuration is your diesel ... think these come in various configurations ranging I believe from about 160hp - 250 plus.

Also, do you really get full spec rev. on this engine ?

<hr width=100% size=1>Alf

 

[Tomsk]

The engine is 185hp, revving to roughly 3200rpm, max rev's per spec are 3800.

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Divemaster1]

Next ...

Trust she has a clean bum and do pull these revs in neutral...

And that she is very nose up, trying to get on top of her bow wave..

<hr width=100% size=1>Alf

 

[Wiggo]

OK, so where are the missing 600 rpm, then? Either the engine's down on power or massively overpropped...

<hr width=100% size=1><A target="_blank" HREF=http://homepage.ntlworld.com/graham.wignall/boats/eulimene600x800.jpg>The old and the new</A>

 

[Tomsk]

She only went back in the water last Friday after full bum wipe and paint - so no problem there.

In neutral she actually revs over the 3800 to just under 3900.

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Wiggo]

Overpropped, then, IMHO. If I remember, each two inches off the prop pitch should get you 200rpm, so you sound three sizes too large on the prop. I'm guessing that that would get you into very odd territory, so I think a gearbox ratio change may be the way forward.

<hr width=100% size=1><A target="_blank" HREF=http://homepage.ntlworld.com/graham.wignall/boats/eulimene600x800.jpg>The old and the new</A>

 

[Divemaster1]

Sounds like over propping big way, or that the stern drive is not trimmed in correctly.



<hr width=100% size=1>Alf

 

[Tomsk]

Over propping is what I beleive, but if I reduce any further I think I will lose the whole top end of the power.

What do you mean by trimming the stern drive? The angle of the prop? If so I did wonder about this with the deisel engine being heavier therefore lowering the stern.

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Divemaster1]

Sorry, presumed you had the engine connected to a stern drive as most Bayliners in that size range did / does... if you do, the installation of the heavier diesel engine may have changed the dynamics somewhat.... On the older drives, without hydraulic rams for trimming, you could adjust the drive in or out by moving a bolt behind (forewards) of the drive to a higher or lower position. A lower position would make the prop angle lower, and hence pushing the bow down earlier, letting the hull rise on the bow wave, rather than settling on the stern struggeling to get up and over, with the bow up in the air... If you "dig" the stern down, this may cause some of the revs to be lost... but not completely sure if it would represent 600 RPM, so still could be over propped.

<hr width=100% size=1>Alf

 

[Tomsk]

NOw I'm getting myself confused... ! Yes it's a sterndrive with the usual hydraulic rams for lifing and lowering the prop/leg.

Because of my theory on the weight of the 'lump' I do tend to lift the leg slightly from it's lowest position..

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Tomsk]

And this of course brings me back to my original post (!) - though not the solution - but would larger trim tabs help to lift the stern therefore pushing the bow down a little further?

Every tiny improvement will be better than none!

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Alistairr]

Have you tried it with the trim tabs fully up??
Cause remember Bigger Trim tabs will cause more drag,Hence less speed, And i think more speed will help it get over the hump!
So i'd try it with leg fully down and Tabs full up.



<hr width=100% size=1><A target="_blank" HREF=http://arweb.co.uk/argallery/alistair?&page=1> My Pictures, Look if you want.</A>

 

[Wiggo]

If the leg is the original leg from when the boat had the petrol V8, then the gearing will be all wrong. The 5.7 petrol would rev to at least 1000 rpm higher than your present engine.

You need to trim the leg in as far as it will go to get over the hump, BTW, then trim out to reduce the wetted area once you are planing. Larger tabs will help, but I don't think they're the whole answer, by a long way.

<hr width=100% size=1><A target="_blank" HREF=http://homepage.ntlworld.com/graham.wignall/boats/eulimene600x800.jpg>The old and the new</A>

 

[David2452]

The 2566 is a heavy boat even for a 5.7l 260hp normally fitted, especially if it is the wide body 11' 6" beam but at least the beam gives scope for wider tabs. The original tabs fitted were a little on the small side, the formula is at least one inch of trim tab span (per side) for every foot of boat length. (Examples: 22 foot boat = no less than 24" x 9", 36 foot boat = no less than 36" x 9")” if you fit tabs with the drop edges it will help with low speed wander as well.

Trim the drive right in for getting over the hump and if you do, only then trim out until the revs climb

I suspect further prop changes are not the way to go, the original Alpha 1 drive (unless it was changed during repower) is entirely wrong both in gear ratio and beefiness for the torque and power curve of the motor you have, so have it checked. With all that torque going through it the need to change it may come sooner rather than later anyway.

You may after all this get her to plane at 15-16 cruise if you are lucky.

David


<hr width=100% size=1>

 

[Tomsk]

I figured that re-gearing / changing the leg was going to be the ultimate solution - it's just a cost that I can't bear at the moment so I'm trying to squeeze a little out of everything else until I can...

But thanks all for the advice!

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[cjrvernon]

ive just taken out a 5.7ltr 220hp v8 petrol
and put in a 6.5ltr 190hp v8 diesel
i have a twenty four foot cruisers international
i was using with the petrol a 15x17 prop
but i was told to change the prop to a 15x21
which is a lot diffarent pitch

my boat will be going back in the water in two weeks
but im told this will be fine
all i can do is try and hope they know what they are talking about

<hr width=100% size=1>http://www.cjrvernon.com

 

[cjrvernon]

ive just taken out a 5.7ltr 220hp v8 petrol
and put in a 6.5ltr 190hp v8 diesel
i have a twenty four foot cruisers international
i was using with the petrol a 15x17 prop
but i was told to change the prop to a 15x21
which is a lot diffarent pitch

my boat will be going back in the water in two weeks
but im told this will be fine
all i can do is try and hope they know what they are talking about

<hr width=100% size=1>http://www.cjrvernon.com

 

[Nauti Fox]

Not sure about changing the prop as if it does get on the plane the engine may rev out,ours used to gain about 500 rpm once on the plane.
Al.

<hr width=100% size=1>No dear,the water goes in the other one.

 

[Tomsk]

I think you may be right about that - that's why I am trying to find every other little way of helping her over the lump..

<hr width=100% size=1>Tomsk -

What the hell... it's only money!