Angle of outboard shaft for level hull??

[Robert Wilson]

My 14ft "Orkney" style dinghy squats stern-down with the Seagull / Selva long-shaft 4HP outboard. More powere pulls the stern down further.
To level the hull (stern upwards/bows downwards) should I rake the shaft closer to the transom, or outwards away from the stern - or even true vertical at rest?
I have sat and looked at it, pondered over the problem and can't get my brain in gear on this one.

Any advice?
TIA
RW

 

[Rigger Mortice]

Closer to the transom, but whether that improves performance or not is another matter.
It may also be worth playing around with moving weight (including you!) forward.

 

[VicS]

My 14ft "Orkney" style dinghy squats stern-down with the Seagull / Selva long-shaft 4HP outboard. More powere pulls the stern down further.
To level the hull (stern upwards/bows downwards) should I rake the shaft closer to the transom, or outwards away from the stern - or even true vertical at rest?
I have sat and looked at it, pondered over the problem and can't get my brain in gear on this one.

Any advice?
TIA
RW

My Seagull Owners Handbook says that the engine must be vertical and at the correct height with the exhaust outlet an inch or so below the surface.

If you adjust the rake outwards it will tend to drive the stern down and lift the bow.
If you rake it inwards it will tend to lift the stern and lower the bow.

 

[Lakesailor]

More to do with the weight distribution I fear. I fitted remotes to my 15ft similar dinghy.

Try a long extension on the tiller.
(Is it a displacement hull? Some are, some aren't)

At 2 knots

At 5 knots (even with leg trimmed right in)

At 5 knots with pilot forward. All is serene

Here is a comparison. of trim with weight distribution moved. Even though it was a longshaft it still came up too high with me forward.
I had to cut a 4" notch in the transom to solve that.

 

[Robert Wilson]

My Seagull Owners Handbook says that the engine must be vertical and at the correct height with the exhaust outlet an inch or so below the surface.

If you adjust the rake outwards it will tend to drive the stern down and lift the bow.
If you rake it inwards it will tend to lift the stern and lower the bow.

Well in that case, I think I need to have it slightly inwards. I may have a problem with exhaust as you advise but I'd be able to "lift" it a bit by sitting further forr'ard as Rigger and Lakey suggest. It is a very long shaft.
The whole reason for getting this boat was to have a cuddy (cabin) to protect me (well,SWMBO actually) on windy crossings to Khamsin. It's a mile and a half of open Loch and can get quite nasty. So sitting further into the cuddy should help all round.

The hull section is quite deep and displaces a lot of water but I'm happy to sacrifice speed by staying dry! Certainly I don't expect to get anywhere near planing! I have just bought a long tiller-extension which helps a lot, even though the plasticy clampy bit is weak and bendy. I'll have to beef it up somehow.

Thanks all.

Fair winds to you

RW

 

[Tranona]

It is a very long shaft.


Thanks all.

Fair winds to you

RW

Herein (I think) lies your basic problem. Pretty sure your boat has a 15" transom for a short shaft engine. Using a long shaft on it will lead to exactly the problem you describe!

Solution. Sell the engine to somebody who needs a LS yacht auxiliary and buy a short shaft motor.

 

[Robert Wilson]

That's interesting because I tried my Mercury 4HP which has a slightly shorter shaft and it made no difference. I also have a short shaft Honda 2.3HP but haven't tried it, yet.
The Mercury's propeller doesn't extend below the keel-strake (forgotten the correct name for this -the long woodenny thingy running from the bow to stern!!), whereas the Seagull's does.

Any thoughts on that?

Thanks.

 

[Tranona]

Measure the distance from the top of the transom to the static waterline. 15" is standard, 21" or so long shaft.

As others have said, weight distribution is important as there is little buoyancy in the stern to take the weight of the helm. Prop too deep will dig the stern in more and give you problems with grounding in shallow waters.

 

[Lakesailor]

This from Tohatsu (Nissan).

It's easier with a planing hull to measure this as rowing boat type hulls have variable heights across the transom.

This, for instance, looks OK. But I have made a bracket to lower the engine another 3". Whilst you are in the stern steering it should work as is. But should you go forward when the engine is running or have a boat load of passengers you can't be sure.

 

[Robert Wilson]

Nothing is straightforward in this world!
From Transom top to bottom of hull is c.18 1/2 ", then the "keel" goes down a further c.6".
The bottom of the cavitation plate is half way between the two. i.e. 3" below hull/3" above keel.
The seagull manual states that if the exhaust is too deep there will be problems starting and running. If I sit in the stern, it IS too deep, if I sit in the middle/forward it's much better.

I think, on balance reading your advice(s), I have a half-way house and shall have to put up with it. Sitting forward using a tiller extension certainly helps; I haven't tried extra weight/passengers aboard yet.

Many thanks one and all. If any further thoughts come to mind please let me know.

RW

P.S. out interest, and for future reference, where is the shaft measured (from / to)?

 

[VicS]

Nothing is straightforward in this world!
From Transom top to bottom of hull is c.18 1/2 ", then the "keel" goes down a further c.6".
The bottom of the cavitation plate is half way between the two. i.e. 3" below hull/3" above keel.
The seagull manual states that if the exhaust is too deep there will be problems starting and running. If I sit in the stern, it IS too deep, if I sit in the middle/forward it's much better.

I think, on balance reading your advice(s), I have a half-way house and shall have to put up with it. Sitting forward using a tiller extension certainly helps; I haven't tried extra weight/passengers aboard yet.

Many thanks one and all. If any further thoughts come to mind please let me know.

RW

P.S. out interest, and for future reference, where is the shaft measured (from / to)?

Based on those measurements I think you will need a long-shaft (20 or 21") although you have not given the measurement from transom top to waterline which Tranona referred to.

If starting is difficult, and I have never found it to be so with my Seagull over immersed, send the crew forward while you start it.


Shaft length is from the top of the transom ( or where it will be in the mounting bracket) to the antiventillation plate or some ill defined point a little above it which corresponds with the water line.

Short, aka standard, shaft models are about 15 or 16", long shafts 20 / 21 " extra long about 25 "

 

[Lakesailor]

The bottom of the cavitation plate is half way between the two. i.e. 3" below hull/3" above keel.

P.S. out interest, and for future reference, where is the shaft measured (from / to)?

See the diagram above.

Ignore the skeg of the keel in your deliberations.. The cavitation plate should relate to the bottom of the transom. On the Yamaha I just bought (same as 2 stroke Mariner) the water level should be 3-4" above the cavitation plate.

 

[VicS]

Since my earlier post I have done some measuring... to add to the confusion

On one outboard the distance from the transom top to AV plate is approx 2½" more than the nominal shaft length. On the other it is about 1½" more.

A common diagram is

but I think having the AV plate inline with the transom bottom is really most applicable to fast planing hulls


Another factor to bear in mind is that the water pump must not be starved of water. Some older engines have to be mounted quite deeply to satisfy this requirement.

 

[Robert Wilson]

Confusion comes easily nowadays, I find!
Thanks for the diagram, which makes me realise mine is probably down a bit too far. But at least it won't be starved of water!
Another point I have noticed is that as I give it more power the stern squats lower and the bow comes well up (if I'm not well forward), but I suppose that's normal, especially on non-planing hulls.

 

[Lakesailor]

Yes. Look at the 2nd and 3rd pics I posted. The weight aft is fooling the boat into thinking it can climb onto it's bow wave, like a planing boat. It'll never happen.

 

[Robert Wilson]

Yes. Look at the 2nd and 3rd pics I posted. The weight aft is fooling the boat into thinking it can climb onto it's bow wave, like a planing boat. It'll never happen.