A Photo & Discussion Forum for Wharram Design Enthusiasts
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.
Thats excellent! thanks for taking the time to post the pic up. Can see what you mean now. Dont worry about dimensions, can easily rig mine up...'Papayo' is all dismantled and tarped up for the coming winter now, but the center decking is easily got to so as to get some dimensions..could easily drop my motor some 5 to 6 inches down which would be ideal.
One thing that has given me pause about using a sled on a Tiki 21 is that the deck clearance is lower than a 26 or 30. The sled might be dragging in the water the whole time.
I dont have a photo of the old sled. It was just a long "L" welded up from steel L section laid on it's back and hinged on front with ply bottom and transom - no sides. The arms were long as a short sled is heavy to lift due to leverage. This shape allowed it to lift up until almost flush with the cockpit floor with room for the engine to tilt up behind the cockpit.
The reason I changed was mostly access for starting / controls / etc. and I was building a new cockpit anyway. The shape of the new fixed mount can be seen in my photos. About 5ft long and almost a JWD hull crossection. Pretty much as if I chopped off a section of bow ! Only 8" [20cm] clearance it must often hit the water but I have never noticed it happen so the "V" shape must sit in very smoothly. Great improvement in protection and keeps the leg / prop out of the water much better so no extra drag overall. Much quicker to deploy. Ideally I would love to find a remote tank to sit in the mount, forward of the engine ,keep it all together, but mine is just a very little too wide !
I will revise the mount system, I can easily design it to be adjustable to get the optimum height.
I'm in the process of building new platforms for my Tiki 21. I've got a new Honda 5 and have seen your pictures showing a your engine. Have you tried lowering the transom? and do you know what height it is below the deck today?
I'm thinking of having a box in front of the engine well with a hatch to get the fuel can out of the way. Any thoughts?
If you have a photo showing the platform cut out shape that works for your honda, I'd be please to see it.
I have exactly the same issues with my 21 and have also considered taking an inch out of the transom. My outboard is a long shaft 5hp Honda. It looks like the designed outboard height is a bit high. I think that Galway Bay's suggestion of a sled would work, but haven't seen one on a 21 yet. If I was wealthy I would get the extra long shaft Tohatsu (25").
I'm not sure that my ransom is to plan - I think it is, but can't say for sure. If it is to plan it needs to be at least one inch lower, though 2 inches would be better. If I was doing it from scratch, I would set the outboard so that it is a hairsbreadth above the lowest point of the keel.
Thanks for the input. Your motor looks a bit higher than I'm planning for mine. I'm wondering if you're getting the cavitation plate 4" below the water surface as recommended by Honda?
The problem with lowering the transom is that as you drop the level, the size of the hole in the platform needs to grow in front of engine. This allows the engine case to dip into the hole as the motor tilts forward. It would be nice to avoid a foot sized hole to fall into at a critical moment.
Another design consideration is that clearance underneath. I'm not sure how big an issue it is for coastal cruising, but I want to avoid water slapping into the motor/transom when under sail.
P.S Keep up the detailed blog, it's really helpful.
My cavitation plate is not 4" below the water surface, so it sounds like it definitely needs to be lower than mine. The motor will be sunk into the hole, but it would be worth it to avoid the ventilation issues.
For what it is worth guys and remember I do not sail a T21 -
It is possibly not necessary to lower the whole transom to get the extra depth. A small "Pod" under the existing cockpit / deck will allow the O/B to sit lower and if this has an angle of approx. 60 at it's base it will cut through any waves without any slamming. This is what I now use and consider it better than the sled I had before. My pod base is only 8" / 20 cm above the water but it never slams.
That's useful information. I now have less concern over my plan to slightly lower the transom and increase the size of the box structure under the central platform to take the fuel tank. This structure will have a slope at the from at around 60 degrees.
I also had the idea of putting the tank in the pod. It would be a very tidy arrangement. I even toyed with the possibility of putting the gas bottle there as well. You may have to get a tank to suit. I did not manage it ...but am very close and I may see a tank to fit. My pod is a triangle in cross section 24" wide on top and would be 20" deep but I cut it off at 16" and have a small 4" wide bottom board. It also tapers forward so very boat shaped. To fit a tank I think you will need to consider a cross section like an upside down house gable - ie side close to vertical and bottom panels meeting at approx. 90' Even so you will probably need a small tank.
But has anyone out there sealed up a space like this to line it as a tank ? Or made a tank to fit ? Easy enough for water maybe but for petrol ?
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.
I'll post final designs when it's all done. Comments welcomed.