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As you can see on the pictures, my engine is too high. I have a 5 hp Yamaha with long shaft. The center section are designed according to plans. It looks like I can cut 8 cm in the transom to get it lower. Can anybody see any negative consequences doing that?

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I wonder ? remember when the boat has a bit of a load on it,the motor will sit deeper. Other wise cut away some of the bulkhead, but make sure that when you pull the motor up to it's resting position it has enough room to do so. ie the front of the engine cover does not hit the cockpit floor.

I hope i have not confused you.

If you are worried about the strength of the bulkhead you could put some triangular webs in to transfer the load in to the deck

Have you launched the boat and tried the engine in the water ? It is not clear to me from the photo that the motor is too high. In fact it looks about right.  This boat draws maybe 40cm ? Outboards are not in my experience designed to run so deep. I have had problems with a deep set-up due to excessive back-pressure in the exhaust. The deeply immersed leg will add to drag making the motor less efficient. These motors give best performance when set to run at the designed depth below the water regardless of the depth of the hull they are fitted to. From your page it is not clear if you have launched yet. If not launch the boat - you may be solving a problem that does not exist. I have a longshaft on my P31. It works perfectly with the cavitation plate 6cm or so below the surface as the manufacturer intended. Also the motor can be run in very shallow water without fear as the keels rest on the sand / bottom long before the propeller is in any danger.

Good point about being clear of the lowest point of the hulls, sods law will mean that there is a lump in the bottom between the 2 hulls when you touch!  On my hinemoa the engine was originally placed well aft and used to suffer from cavitation because of hobby horsing, when I moved it forward to a simlar position to that on the Tiki designs the problem went away.

Galway Bay said:

Have you launched the boat and tried the engine in the water ? It is not clear to me from the photo that the motor is too high. In fact it looks about right.  This boat draws maybe 40cm ? Outboards are not in my experience designed to run so deep. I have had problems with a deep set-up due to excessive back-pressure in the exhaust. The deeply immersed leg will add to drag making the motor less efficient. These motors give best performance when set to run at the designed depth below the water regardless of the depth of the hull they are fitted to. From your page it is not clear if you have launched yet. If not launch the boat - you may be solving a problem that does not exist. I have a longshaft on my P31. It works perfectly with the cavitation plate 6cm or so below the surface as the manufacturer intended. Also the motor can be run in very shallow water without fear as the keels rest on the sand / bottom long before the propeller is in any danger.

Thank for feedback. I am just a week from launching so I think will follow you suggestion and test it first. I have enough problems to solve.

 

Always great to hear of someone finishing and launching. Fix what you must and don't go looking for problems. In time they will find you...

Safe Seas.

Morning,

Found this:

http://www.pinoyboats.org/forum/search.php?keywords=Transom&t=9...

"OB Transom. Two laminations of 3/4" thick ply. Got to read the PCA (Polynesian Catamaran Association) on the net and it mentioned to do a cutout on the transom to set the engine a little lower in the water. I followed the advice and cut a 3" off the top. the transom is angled to allow the OB lock at full up to engage without hitting the aft crossbeam. Plans do not specify this. TIKi 21 owners having vertical transoms keep their outboards up by lashing :cry: "

It seems that the guy just cut down the transom a little to have his outboard, not only lower, but to be abble to pull it up properly.

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