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Well, got my Tiki 21 'Papayo' out of the water and dismantled to be covered up for winter..which is fast approaching my part of the world.

 

My Outboard Motor cavitates when I am motoring in choppy seas. Its a Yamaha 4stroke at 4HP, its powerful enough and I suspect its got the 'long shaft' option fitted. I am planning on removing about an inch from the top of the transom as the motor could then tilt up further and engage its final lock. would save its prop being dragged in the water. I dont like lashing the motor up in case I need to drop it in a hurry..ie a botched change of tack too close to shore :)

 

Seems to me that the prop could do with being a good 6 inches further down, that way in the choppy water it wouldnt cavitate so much, if at all.  Its annoying as it is because at times you need forward motion rather than having to throttle back and letting the cavitation subside.

 

 

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Love the drawings Ian !  What CAD system did you use to create the design ?

Rob

Ian Bamsey said:

This is in line with my thinking. There are a few considerations. The platform with its box section acts as a beam. The top platform is in compression and therefore cutting out the hatch to allow the tank in will weaken this structure. Some additional reinforcing may be required.

I've not completed the design yet, but the models below show my thinking for the central platform structure on the Tiki 21.

1. On the top the green shows the thicker ply lamination at 12mm, the remainder is 6mm. The transom cut away to lower the engine is not shown. You can see one tank which is a model of my 10 litre. The box should take a 12 litre as well based on dimensions of an available plastic tank.

2. This is from below with the base removed so show detail. 

3. Same again but more straight on to show the struts to strengthen platform around the hatch.

The additional weight is small as it comes only from the larger area of 12mm on the top, plus two more struts and a little more base size in the box. 

I'll post final designs when it's all done. Comments welcomed.

Hi Ian,

One practical issue is that when you are sailing the boat you will be constantly crossing back and forth in front of the outboard - it is the busiest bit of the deck. A big hole there is going to be a pain. I tie off my gas tank next to the o/b or on the rear deck and have never had any problems even in foul weather. Another thought is that you can just cut the standard transom lower to get more prop depth - it would be way easier. I have found that I never really need to turn the o/b even though the boat lives in very tight marina space - in my humble opinion this means that you only need to cut the deck to fit the o/b - not the swing of the o/b. As Galway Bay would say - keep it simple and go sailing.

Roger

Thanks Roger,

The issue is that by cutting the transom lower (to get that 4'' for the cav plate under the water) means the Honda 5 cannot tilt up out of the water easily. The front of the engine cover fouls on the front edge of the engine well. With the engine higher this is less of a problem. I'm trying to work out the tightest engine well hole shape for the engine that I can.

Useful info regarding the need to turn the o/b. I've found the same with the old engine, and so I wont make the well any bigger to accommodate the turn.

The reason I'm going to trouble of building a box for the tank is I'm force to rebuild the platform as the existing one is in a bad way and the work involved in renovation would be comparable with the rebuild.

I'm looking forward to getting back in the water ... but will use the wait for the spring to renovate.

The CAD is a 3D modeller called Blender (it's free) It takes effort to get going and it's not like ACAD, but I've used it successfully to design a new cut roof for chalet, so it work well!

Ian

Robert Hughes said:

Love the drawings Ian !  What CAD system did you use to create the design ?

Rob

Hi Ian,

Its true that it will hit the standard cut out in front of the o/b, but it is easier to make this hole a bit longer to accommodate the o/b cover than to make a more complex fix. One of the swings and roundabouts of dropping the height of the transom is that the prop end will swing higher up under the rear beam and will not drag (as it does in chop at the standard height).

Roger

Ian Bamsey said:

Thanks Roger,

The issue is that by cutting the transom lower (to get that 4'' for the cav plate under the water) means the Honda 5 cannot tilt up out of the water easily. The front of the engine cover fouls on the front edge of the engine well. With the engine higher this is less of a problem. I'm trying to work out the tightest engine well hole shape for the engine that I can.

Useful info regarding the need to turn the o/b. I've found the same with the old engine, and so I wont make the well any bigger to accommodate the turn.

The reason I'm going to trouble of building a box for the tank is I'm force to rebuild the platform as the existing one is in a bad way and the work involved in renovation would be comparable with the rebuild.

I'm looking forward to getting back in the water ... but will use the wait for the spring to renovate.

Here's a picture of a kiwi tiki 21 that has had the o/b sunk down - I can't remember where I got it, perhaps the owner will recognize it and comment. But it looks workable to me.

Roger

Fascinating Roger! Check out that semi-circular plate in front of the engine. I wonder if this is removable to allow the engine to tilt forward? I was thinking of a possible solution like this. And that is about the depth you need the engine if it's a 20'' outboard.

Hi, the photo of the above outboard looks similar to mine. Mine is the longshaft version. I am in the process of putting my Tiki back together after last winter. It might just get sold but if the lad who wants to buy it wastes my time again I will keep her for sure, so will invest some time to lower the motor. 

 

Toying with the idea of lowering the transom cutout to lower the motor another inch or so but must admit the rear beam mount is very attractive. Mostly that I don't have to butcher the transom and might even use that space to place an external fuel tank.

 

I shall get welding and see what I can come up with:)  main criteria is keeping the motor high enough to not get it wiped out with a wave between the hulls.

Hmm, quite a thought provoking subject.  My latest idea is to build a completely new central deck. Planning on lowering the transom top edge by a good 4 inches or so..and deepening the void so the motor can still be tilted up while sailing.

 

Meanwhile I plan to build an extension to the existing transom to bring the motor to the rear of the aft cross beam..and plan to make this adjustable to find the best depth for the prop.

 

A consideration is ease of use, plenty of kelp to cross where I launch from, the motor needs to be tilted up to chop the kelp up..and also to clear the cooling intake. A flap of kelp makes an ideal seal for small intakes :)

 

Will post up some pics and results of my experiment.

 

OR!! why not a small engine and a great long shaft and prop, like the 'outboards' of the Far East :)

 

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