Cavitation - Twin outboards on an Avon Searider

  • Thread starter Thread starter Alcyone
  • Start date Start date
A

Alcyone

Guest
I've fitted twin 40hp Yamaha outboards to my 5.4m old avon searider. When I trialled it on Saturday, I was getting lots of cavitation. A friend says that the engines could be too high.

The cavitation plate on both engines is about 1" higher than the chines on the hull directly in front of them. Any opinions on whether they should be lower, and if so, by how much?

Cheers.
 
When are you getting ventilation (cavitation is something else) during acceleration, or when you turn? Are both engines trimmed down and the props in good condition and the right size checked against a rev counter?


Pete
 
As sson as she tries to plane, which she fails to do because of the cavitation. Straight line. Both props good. Trimmed all the way down (I usually trim when on the plane).

Props are the right size and have been checked. I bought them and fitted them.
 
How much space have you got before the engine saddles touch the top of the transom. I take it these are standard shaft engines rather than long shaft.

If you can't get the boat to plane then props may be the wrong size, certainly no way of confirming they are the correct size.

Is there a reason why you want twin outboards because if it comes to cutting down the transom or changing to a long shaft single, the single will be faster but changing starts getting expensive.

Pete
 
Last edited:
Hi,

They are long shaft engines. I know the engines and props are correct, as they were fine on the previous boat - which I knew well. I bought them as I knew they had been well looked after, and the price was too good to miss.

I've been told that the cavitation plate should be below or, as a minimum, the same height, as the hull directly in front of it. Mine are about 1" above the bottom of the chines.

Any thoughts on this?
 
The 1 inch above normally apply to single engined boats as a start point and adjust from there , indeed rib makers like Osprey built 21" transoms bcause they knew that with a 20" longshaft the engine would be in the right place. Since you have long shaft engines I would drop them down an inch and see what happens. If you go to far down all that will happen is the top speed drops but for now you need to get the boat on the plane first.

Just a thought these are 2 strokes aren't they or have you got heavy 4 stroke engines?

Pete
 
Last edited:
Yes, 2 strokes. I think I am going to have to cut down the transom. I want to try and only do it once if I can. I'm not overly concerned about top speed - it's for diving.

Thanks for your help.
 
Please don't. As a dive boat you are carrying a heavy payload and it used to astonish me the kit people think they need, plus most of them have eaten to many pies ;) The result will be water sloshing in over the transom at rest and when you stop. Your twin 40s will only equal a single 60hp engine btw.

Instead sell the 40s and buy a single 90 two stroke. The Avon 5.4m with a single 90 (Yamaha if you can find a good one) is a superb set up and the Yam 90 should run like a sewing machine. You won't need anymore power than 90, although there are some 5.4s with the Yam 115 V4 and strengthened transoms. This would be a wicked piece of kit for 4 divers. Tow it anywhere, easy to launch but very seaworthy.

Pete
 
Not an option, I'm afraid. I need to make this setup work.
 
understood, lower them down are far as they will go and trim hard in (without the pins). Take it for a spin with a couple of big lads in the bow as see how you get along.

Pete
 
Pete 7 has said an awfull lot of correct stuff Alcyone.
Inc Cavitation and Ventilation.

The Searider is a good hull/rib.

From the info given.
The eng / engines are too high.
imho
 
OK, the boat is an Avon sea rider, commercial spec, late 1970s. I bought her as a bare hull with trailer. I had intended to put a single engine and 4hp (say) backup. I had the chance to buy a pair of 40hp yamahas, with known history and very well looked after from a friend at a very good price. Not what I wanted initially, agreed, but I've seen plenty of seariders with similar set ups.

I really appreciate the fact that a different set up might be better, honestly, but I have what I have, and need to make that work.

What I'm really looking for is someone who knows how you set the height of twin outboards on a V-hull. As I say, I've been told that the anti cavitation plate should be no higher than the hull immediately in front of it. Mine are about 1" higher. I need to lower the engines, I think by maybe 3", but I was hoping somone might have some definitive advice/knowledge.

Again, I really do appreciate people taking the time to reply. Thanks.
 
Has your Avon got the flooding hull? that would add quite a bit of weight to the stern and not help you get up on the plane until all the water has drained out. For dive boat use I would block up the drain and the breather holes in the bow. Also worth measuring the engines to confirm they are longshaft at 20" and not something in between (Mariner did a mid length 40hp for the RNLI).

This might be worth a read:

http://rib.net/forum/showthread.php?t=23159&highlight=longshaft+measure

Pete
 
Thanks Pete. I am slowly getting there. It does seem as if level or below (up to 2") is correct.

Yes, my hull is flooding, although I do intend to block it, as you suggest. It did not affect these tests, however, as I felt the hull drain a number of times when trying to plane.

In this picture:

http://rib.net/forum/attachment.php?attachmentid=32757&d=1202590056

The two chines either side of the hole are just below my twin engines. I think I will need to cut my transom by about 3" to get the cavitation plates 2" below those chines.

I've not yet sen anyone say don't do that, and it certainly cavitates badly at the moment, so I reckon that's right.
 
You must log in or register to reply here.

Share this page

Top