Dont know how deep to have my outboard!

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matnoo

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I have bought an outboard on ebay, this one to be exact a yamaha 9.9, 80s/90s model

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?V...58055%26fvi%3D1


I am restoring my boat and need to replace the transom bracket, which is no problem, but i need to know where should the waterline be?!

There is a cavitation plate, then 4 inches above that, another plate, and 4 inches above that, another plate (you can just about see them on the main photo)

Am i right in saying the water line needs to sit inbetween the two upper plates?

For the transom, im CNC machining it out of 50mm thick marine ply, bonded together with pva based glue, screwed with long stainless screws and bolted with M10 boltsthrough the fibreglas hull in 6 places. Is this adequate, I recon the motor is about 50kilos, 7stone

Mat
 
Particullarly if it is a planing hull i beleive the cavitation plate should be level with the bottom of the hull.
 
[ QUOTE ]
Am i right in saying the water line needs to sit inbetween the two upper plates?

[/ QUOTE ]NO! - The lower plate in the photo - the one just above the prop - is the (anti) cavitation plate and this generally should be positioned level with the bottom of the transom or slightly below (~1"). The owners manual will give better details. A copy of the awners (users) manual will be available on the yamaha website.
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I would imagine the depth is not critical as long as its at least under the bottom of the hull but get other opinions as im not sure.
 
[ QUOTE ]

The boat is a fin keel yacht, the bottom of the hull at the back is only about an inch under the water, ie it sweeps gently like this one

http://www.radford-yacht.com/dsn058/dsn058c3.jpg

If i put the cavitation plate level with the bottom of the hull, when a wave come along it would be clean out of the water?

Mat

[/ QUOTE ]

Like many other boats ...... it is a problem that you learn to live with ... you develop little tricks like turning at an angle to waves etc. to get a half roll / pitch motion to reduce prop time out of water etc.
My last 2 boats with transom hung o/boards ... I put them much deeper than manual said ... literally bottom of mount ply-board was only mm above water ... but then I had lifting brackets. My last one ... a 7.5 Merc Sailpower O/board ... when driving boat .... water would be splashing up between the thumb screws holding engine on ... But I am talking sail-boats.

Mobo's - keep to other's posts of the cavitation plate just slightly lower than transom ... as the boat planing should look after pitching to a good degree.
 
You should be able to transfer the water line from the hull to gauge the expected height of the transom above the water line at the stern. From this mount the bracket accordingly. If in doubt I would always go deep (as per Cliff) i had a 4hp ob that was too high in any short chop the prop would lift and i had to change the drive pin very often (not nice when forced to anchor or heave to whislst dragging the bleeding thing inboard to effect the repair! Lowering it was better but when underway the stern digs in and when throttled up the bracket would become sweatingly close to the water, had great fun balancing speed against trim!
 
Hi,

My Yam 4 stroke 9.9HP (1992) sits in a well at the back of the cockpit, just in front of the transom.

This picture shows where the waterline is. Mine looks a bit different from yours (prob mine is older?) but may be a guide. I have no probs with too much exhaust back pressure, and never comes out of water (but it is in a well not on transom)

Outboardlegsmall.jpg


May or may not help, but there you are.
 
In theory you should set the bracket so that its top edge is the same height above the water as the quoted "shaft length" for the engine. I dont know the exact figures for Yamahas but standard shafts are usually about 15 inches, long shafts are about 20 inches and extra long shafts are about 25 inches.

I cant tell from the photo what your engine is. A standard shaft or a long shaft but the vendor describes it as a long shaft.

However as sbc points out you end up with the prop lifting out of the water in any sort of chop. The solution is to fit it deeper. I have fitted a long shaft engine (20" Evinrude) on a bracket that is about the "correct" height for a standard (15") shaft engine. I do not have any problems starting it at that extra depth and the prop stays in the water. I assume we are talking about sailing at sea rather than a big duck pond like Windy Mere


I suggest therefore that the bracket should be fitted so that its top edge is 15 or 16 inches above the water line when lowered (Assuming that your engine is indeed a long shaft)

You might be better off on a boat that size with an extra long shaft (25") fitted on as though it were a 20" long shaft model.
 
You will also be better off with a different prop to the standard one. For a sailing boat you need a smaller pitch combined with a larger diameter or bigger blade area that you do for a fast runabout. IIRC Yamaha do high thrust props for sailing boats etc so talk to your dealer. You would also be better off with an engine that does not take the exhast gases throuh the propeller hub. With it like that you get very poor performance astern as all the gases are sucked back into the blades. My Evinrude has the exhaust outlet between two cavitation plates well above the prop.


50 kg is heavy for a 9.9 long shaft. The Evinrude one is 35.5kg and 40kg for the extra longshaft "sail" version
 
[ QUOTE ]
I assume we are talking about sailing at sea rather than a big duck pond like Windy Mere


[/ QUOTE ]

Oh yes. I forgot about that.

Feb01.jpg



Feb03.jpg
 
All the instruction books will tell you, "Ant-Cavitation plate should be a Minnimum of 1inch below water level. Mentioned before, lower and you get less air time when you come off the top of a wave, BUT be aware, too deep and you will drown the bugger.
 
The picture in the link is of quite a big boat for an o/b I expect your boat is smaller.
The pitch characteristics of the boat make a lot of difference to your depth decision.
My 21fter is quite light and has huge tansom depth position between no one on board or a person on the bow (working anchor or mooring) with transom way above the water to the other extreme of 4 people in cockpit when it drops by nearly 12 inches.
So I find even with a longshaft motor that it is essential to have an adjustable bracket. The motor can be raised and lowered by about 10 inches.
So once under way and all crew in the cockpit I raise the motor until the cavitation plate is just below the water level.( for better efficiency ie less drag and easier access) But if there is any large waves then I drop it to ensure positive drive right through the wave passage. Surprisingly I have seen the motor almost dissapear under a wave while running with no hesitation (just my heart).
On arrival at the mooring I drop it right down so that it does not cavitate (lose grip) with people on the bow picking up the buoy.
As suggested to put the motor in a well reduces the pitch problem but takes room away from the cockpit and is not so easy to use for steering. But if you only have a fixed mount make it low. good luck olewill
 
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