Transom height/shaft length on twins

martini · 24 March 2008, 15:00

Can anyone advise me on how to work out the required shaft length on a deep vee hull when using twin outboards?

On a single engine set up its easy, measure from top of transom to bottom of keel and that's your shaft length. But how is the vent plate supposed to sit in relation to the hull when the hull is at 21 degrees from horizontal?

Do you put the vent plate level with the bottom of the hull on the centreline of the outboard, ie. half the vent plate is hiding behind the transom and the other half is in the water flow?

Obviously every boat is different and minor tweaks need to be made to get it just right but where is the start point?

I bought a pair of 225hp Optimax's for my Arctic 28 before I had the boat delivered. I went for 25" (XL) thinking they were too long, the plan being to mount them on jacks to lift them the extra 5". The reasons behind this were:
a) keeps the engines from getting swamped so much
b) allows me to close in the transom cut out so I don't get water spilling over into the boat so much.
Trouble is it's just dawned on me that this won't work! With the motors set back on the jacks by 5.5" and the transom closed in, the hydraulic steering cylinder etc. will foul the transom when I try and tilt! At least I guess it will, I haven't got the motors yet.

So I'm now looking at fitting the motors straight onto the transom and I've been trying to work out how much I may need to raise the transom height. Now that I've taken a tape measure to the boat, I'm now wondering if I have actually bought the correct shaft length and can just bolt them straight on!

If this is the case then I won't be able to build up the transom height to keep the water out but looking at the tide mark on the back, it looks like I have around 8" above the waterline which isn't too bad I spose.

Is this post getting too long?? Hello.....? Anyone one there....? Zzzzzz...

Ps. scanned a sketch I made of the transom, not sure how it will look when it's resized. And no, it's not to scale!

OceanEco · 24 March 2008, 15:17

I would fabricate an outboard extension out of aluminum. Not too many people seem to them in the UK.... They are used in Canada quite a bit. It would give you the chance to build up the transom height and it would give you about another 2' of length to the boat. It would enable you to tip the motors with no problems and get the perfect height. It also gives you more usable deck space. With a fiberglass boat it would need to bolted to the transom as well as using 5200 or sikaflex, I find 5200 much better. If you are interested I can take some pictures of both my ribs which are both done this way. One after market and one at the factory... Just a long thought.....

Pete7 · 24 March 2008, 15:19

Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.

Pete

martini · 24 March 2008, 15:32

Quote:

Originally Posted by Pete7

Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.

That certainly is a good idea, I suppose they might be more forthcoming with info than halmatic themselves!

If by standard shaft you mean long shaft (20"), I'm not so sure. If you look at my drawing you'll see I have 26.5" on the centreline of the outboard. This would be just about right for an XL if my theory is correct.

Pete7 · 24 March 2008, 15:52

You could be lucky, also worth asking them what the minimum distance apart you can rig the engines and what "toe in or out" they recommend because you don't want to mount them parallel.

Pete

martini · 24 March 2008, 16:03

Informative article herehttp://continuouswave.com regarding engine spacing and toe in/out for anyone interested.

Hopefully Turbodiesel will come along soon and enlighten me further

Dirk Diggler · 24 March 2008, 16:05

Quote:

Originally Posted by Pete7

Why not give Barrus a ring later this week and ask what size engines they originally supplied to Halmatic for these boats. My money is on standard shaft engines though.

Pete

And my money would be on you drawing a blank on that one...... you'd get more sense out of Manos.

if your sketch is correct, and you were mounting on those centres, then 25'' (xl) mids would be fine, giving you cav plates 1.5'' above the running surface. After all, it's not as if your putting together a performance boat!

Limey Linda · 24 March 2008, 16:06

Looking at the sketch, I agree with you, 25" XL outboards would be a good fit. I believe that the vent plate should be in clear water. You can always raise them an inch or ttwo later but cannot easily lower them. BTW if you do not have transom drains it would be good to position and put those in before fitting the engines. Your project seems huge compared with mine.

martini · 24 March 2008, 16:13

Thanks Limey, that's 2-1 to the 25" then! You say that the plate should be in clear water, but with 25", more than half of the plate will be out of the water when on the plane would it not?

Limey Linda · 24 March 2008, 16:30

Quote:

Originally Posted by martini

Thanks Limey, that's 2-1 to the 25" then! You say that the plate should be in clear water, but with 25", more than half of the plate will be out of the water when on the plane would it not?

I believe that would be Ok. The alternatives are that the transom was made for XXL engines, but that is very unlikey, and 20 inch would be far too short.

martini · 24 March 2008, 22:01

Anyone want to buy a pair of transom jacks then?

I might have to register at boatmad for this

jwalker · 25 March 2008, 19:22

Quote:

Originally Posted by martini

You say that the plate should be in clear water, but with 25", more than half of the plate will be out of the water when on the plane would it not?

The water rises pretty sharply after the transom and the legs will be a distance back so my money is with Mr Diggler.

Jono · 26 March 2008, 09:29

Quote:

Originally Posted by jwalker

..... so my money is with Mr Diggler.

My vote is for the Tubby One too.. he might sound like he's talkin' bollix.... but he knows a thing or two about building/rigging boats... he's also worth consulting on any question pertaining to yer choice of a good wine too....

pathalla · 26 March 2008, 12:38

You are going to need long shaft motors, I would strongly recommend an outboard bracket, it works very well, doesn't seem very popular on your side of the pond and from a functional, practical point of view not using one is shortchanging yourself and the boat. One, you can tilt the motors ALL the way up out of the water with a bracket with no clearance issues, without one I'll bet you'll have problems. Two, if you ever have problems out on the water a bracket gives you a relatively stable area to troubleshoot problems from without leaning out over the back of the boat, a huge advantage. Three, the bracket increases the effective length of the boat (rarely a disadvantage) and imho improves the performance of the boat due to offset.
As to setting it up I could probably give you some pictures and some measurements regarding motor installation, it might help, I am sure there are others on this forum who also can add plenty of input. Good luck!

I hate to be the lone voice of dissent but I think you may be too short on your
motors. I think they'll work but you might find those power heads closer to the water than you'd prefer. My boat measures 28 inches from transom (bracket lip) to keel, I have about 10 inches of clearance with the waterline at the stern. that may seem like a lot but when the boat is pitching and the motors are hanging off the back it's just enough to keep the powerheads up out of the water in most sea states. 25 may not be enough length on your boat, I hope it is but from my point of view... I don't know... Look, if they are too long raising them five inches is hardly noticed, if they are too short, what do you do?
I have 30 inch shafts on my motors.

Dirk Diggler · 26 March 2008, 19:23

Quote:

Originally Posted by pathalla

You are going to need long shaft motors,

So your saying he needs 20'' mids?

martini · 26 March 2008, 20:06

My local Merc dealer and also Barrus who rigged the boat originally both confirm it's 25" XL shafts so pat yourself on the back whoever guessed correctly!

Limey Linda · 26 March 2008, 20:12

Quote:

Originally Posted by Dirk Diggler

So your saying he needs 20'' mids?

I think Pat ment XL shafts- 25inches. he has 30inch shafts which is XXL.
Very confusing at the mo. martini has 25 inchers coming I believe and the dilima is wether to use the brackets or not. Even a 5 inch set back bracket should have enough clearance for the hyd. steering gear when tilted.

ALANT · 26 March 2008, 20:47

Have they arrived from America yet?

Dirk Diggler · 26 March 2008, 21:16

Quote:

Originally Posted by Limey Linda

I think Pat ment XL shafts- 25inches. he has 30inch shafts which is XXL.

Pat's a fool then! It ain't rocket science, a transom that is approx 25'' high needs motors that are xl (25''), give or take an inch really doesn't matter in this application. As for a standoff box/setback bracket, you yanks are obsessed with them.

martini · 26 March 2008, 22:26

Quote:

Originally Posted by ALANT

Have they arrived from America yet?

They're due in a couple of weeks.

The original reason for using jacks was because I thought my shafts were too long and I could have used the jacks to raise the motors up to the correct height.

Now that I know the motors are the correct length I have the choice of using them or not. However as the jacks I've got are only 5.5" set-back they may still foul the transom. According to the Seastar website, I need 6" clearance in front of the engine mounting in order to tilt the motor fully.