O/B Transom Heights for Sailing Catamarans

gxxdman

Cadet
Joined
Oct 30, 2006
Messages
13
Hi,

My engine is mounted on a transom between the hulls. The cavitation plate is under the water but of course the water level changes between the hulls when small waves pass between them and the hulls themselves do not respond.

My boat is an old sailing catamaran an 8m Bobcat and I am looking for power not speed but without cavitation. How long should the shaft be? The boat transom height is about 20in/500mm.

Using a Honda BF15A Old Style engine with the Long (because Extra Long very difficult to obtain) Shaft (570mm/22.4in) I have a cavitation problem when turning or in a choppy sea. Any ideas - I cannot reconfigure my boat or use a lowering bracket. Maybe there are other similar sized engines with longer shafts in some parts of the world, unfortunately, there don't seem to be many in Europe of a similar hp/spec.

Thanks, John (Scotland)
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: O/B Transom Heights for Sailing Catamarans

If the standing height of the transom is 20 inches.Then you will have some problems.Is the boat set up for sailing?Try various trim angles it may defy logic because of the non planing hulls.you probably need to keep the stern as low as possible.
its possible out trim may actually lower the stern a little.
A 4 blade prop something like they reccomend for pontoon boats.may help.Pushing displacement hulls beyond displacement speeds 6-8 MPH may create ventilation problems.
There are kicker motor brackets designed to raise and lower kickers this may be a way to get the motor a little lower.
 

Paul Moir

Admiral
Joined
Nov 5, 2002
Messages
6,847
Re: O/B Transom Heights for Sailing Catamarans

Have a look at baymfg.com . They sell kits to lengthen a 20" outboard to 25" covering about every model except hondas. I notice marineparts.com/marinemanifold.com are carrying their line.

If there's no way to get your outboard in any deeper, then there's no way to prevent ventilation without making it longer. Normally I would suggest a drop down bracket but you said you can't do that. The 5 extra inches are going to add quite a bit of drag unless you tilt the engine out of the water. Even tilted, you can expect the anti-ventilation plate to catch the occasional wave more frequently since tilted up it's still at an angle.

Around here 25" outboards are not all that uncommon. Evinrude made their 25" yachtwins up until the late 90s, the largest being 9.9hp. I don't know about yamaha's, etc, offerings though.
 

gxxdman

Cadet
Joined
Oct 30, 2006
Messages
13
Re: O/B Transom Heights for Sailing Catamarans

Thanks for the suggestions folks - I shall chase up a few of the ideas. I do not need to go more than about 3-5 knots using the outboard. I generally sail whenever I can but I do need a reliable aux. engine to get in and out of harbour - my sailing area is generally the north coast of Menorca in the Med. it is a very rocky and unforgiving coastline but beautiful for sailing in cat.

A few additional points:

a) I hadn't thought of the trim but was interested in the idea of less pitch/more blades except that I was told that they were not available for Hondas. Is this so?

b) I shall try and find out more about the Evinrude 25" - I hadn't heard about them. Were they 4 strokes, electric start with charger?

c) I have also been told that Honda do not supply kits and 4 parts must be bought separately at a price more than I paid for the engine! Ideas to get parts cheaper eg second hand would be appreciated.

d) Would it be a very difficult job to extend the shaft without specialist knowledge/experience? I have a full workshop manual.

Thanks again for your help - John
 

gxxdman

Cadet
Joined
Oct 30, 2006
Messages
13
Re: O/B Transom Heights for Sailing Catamarans

Once again thanks for your help folks. Having checked out a few possibilities I am wondering about trying a 4 blade prop which was recommended - is it possible to get one of these that will fit my honda? Also would a finer pitch be the best bet since I am not interested in the speed of the boat as such more the power to get me out of difficult situations - this may need me to be able to move throught the water at about 5kts though. Thanks again - John (Scotland & Spain)
 
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