Foils

I have a 1964 evinrde 18hp fast twin longshaft,I intend to use it to power a 13ft fibreglass speedboat from the 60's.
Would it be of some advantage to fit Foils on it at the cavitation plate.The transom is being converted from a short to suit the longshaft motor.
If it is to some advantage,what would it be?
When re-furbing the boat I have kept it as light in weight as possible.
Any advice will be greatly appreciated.
Kind regards

 

Assuming that I have understood correctly what you are describing here, I don't think there would be any advantage to fitting foils in lieu of anti-aeration plate. Foil shapes are basically and generally used when you want to either reduce the drag produced by a blunt-shaped underwater structure or if you want to produce some useful hydrodynamic force while minimizing drag. An example of the former case are the lower units of your outboard engines, and the example of the later use are the vertical lift force produced by wings of hydrofoil boats or the lateral lift force produced by keels and rudders.

But a lift force from a foil also produces a moment, i.e. it tends to rotate or trim the boat if not properly balanced by other forces (like drag, weight, or forces and moments from other foils). So by applying a lifting force at the transom of your boat, the result would be a strong pitch-up or pitch-down moment, which would either bury your bow into the water, or would bury down your transom. In boat cases the result would be very bad for the handling of the boat, and even dangerous at high speeds.
Hope this helps.

Cheers

 

Thanks for the reply Daiquiri,
I think I have used the wrong term when I said "Foils" What I meant was the addition of "Wings" or plates to the cavitation plate on the outboard itself,they are an off the shelf item advertised for sale.
I didnt mean adding anything to the boat itself.I understand what you say about foils altering the trim and balance of the boat.The hull is a good shape and is almost flat from halfway down.My idea of just 18hp is to keep it as light as possible,but still have enough power to deal with wind and tides etc.I am converting the boat to use for close inshore fishing,High speeds are not important,but more the economy and safety.
That is why I was asking about the advantages of additional "Wings" on the cavitation plate.If you have any info it will be very helpful.
Kind regards

 

Can you give more info about that product? Their website, a pic or the commercial name?

 

If they are what I think they are, then they are a waste of money and added wetted area (drag).

 

Hi Daiquiri and sub-tom,
I found the name of those fins,they are-DOEL FINS.
I have sent an advert for them off ebay,I am not computer literate enough to send the link,so I have asked PAR if he will insert it in the forum for me.
Quite a lot of boats in the uk have them fitted.
I get the feeling these might be "Old hat" to you guys.
My initial enquiry was to get some experienced points of view on their use.
Kind regards

 

Found the product page: http://www.davisnet.com/marine/products/marine_product.asp?pnum=00440
They work as I thought, push the bow down, and hence will imo have the pros and cons I said. At high speeds pushing the bow down might be dangerous.

Checked for more info. In this forum: http://www.ybw.com/forums/showpost.php?p=2191993&postcount=7 the only guy who gives some more detailed description of his experience says: "When I opened the throttles fully they gave so much lift that the whole boat started to dance/waggle". That's exactly the expected consequence of what I've described in the first reply.

However, a faster hole shot could be an advantage, so IMO the best thing would be to have trimmable fins. Or - the trim tabs...

 

That puts me in the picture,thanks.
The quicker out of the hole appeals,and more so throttling back and staying on the plane.The boat doesn't warrant the expense of trim tabs,and I wouldnt know how to fit them.
I think if I fit the Doel fins I will pay attention to using them at high speeds and turns etc.like you advise.I cant see the boat doing much more than 20-25mph at full throttle.
I might set the engine an inch overdepth,what do you reckon?

 

Well, there you go.

I hope they work out for you.

Be sure and post your findings.

 

Thanks Tom,
It could be a while yet,I have been waiting for the weather temp.to change so I can fibreglass the new stringers,deck and transom.Seems like everytime I pop my head out of the door the weather goes inclement.
I have to trailer the boat 70mls to the welsh coast to try it,which makes it pretty important to get it right first time.The weather can be ok when I leave Manchester,but too choppy when I get there.Oh the joys of owning a boat.Haha

 

Changing from a short shaft to a long shaft out board !! do your home work and understand the height the your new motor needs to be set at !!!.
Draw it out full size on a piece of plywood !! The under side of your cavitaion plate on your outboard needs to be level with the bottom of your boat !!! so thats your starting point !!.
Take that measurment and deduct 10mm to be able to drop your motor that amount if need be for some reason !!
Usually the holes in the motor mounting plates are sloted at the bottom and holes at the top so before you get to carried away and start drilling holes !!. Mount the motor and clamp it there and double check all your measurements and that it straight !! (in the vertical plain and in the centre of the transom you would be amazed at how many boats are not done properly !!)
Start first with the outboard plate underside level with the bottom of the hull and thats your first lot of holes !!

 

I understand that is the method to follow,do you have any advice on transfering the lowest point of the hull to the transom.My boat is on chocks at the moment,with the bow higher than the stern.I have thought of using a spirit level,but this would only work if the boat was level in the water.There must be a practised method,a datum point to work from.There is no waterline mark on the boat.

 

Just use the level across from the point of the chine on each side to level the hull athwartsips (Across ) when it level then use the level vertical or a plum bob and stand back aways and just sight it !!
use a straight edge about 3 foot long and push agains the bottom of the boat so it comes close to where the outboard leg will be and then just mount the motor !! clamp it lightly and raise to the point i discribed !!. :idea::?:
Theres a possability the motor could even go up 10 mm! from the start possition depending on how far away from the transom the leg of the outboard is . the further back the motor is the higher it could go !! . The higher it is the lighter it is to steer and the faster it will go !! all about drag !!! reduce drag and performance gets better !!.

 

Thanks T,I can see what you mean.I will measure the distance the outboard extends from the transom and allow for it.I didnt realise it was so crucial [10mm] to get it at the right height.I was going to set it 1" lower to make sure it doesnt cavitate if there'e a bit of a chop.I realise the drag factor is important,and it alters the speed and fuel economy.Would it make a lot of difference to set it 1" lower?With it being a speedboat hull I think it may "wave hop"at speed and bring the prop out of the water,and cause it to over rev.I'm not looking for high speed,but the economy is important.I'm looking for a happy medium really.

 

Hi
In setting up a outboard for height the differance between cavitation and not cavitating can be as little as 5 mm !!
I set a 150 hp extra long shaft motor on the back of a 20 foot boat with a 24 inch bracket and we were able to set the motor more than 55 mm high becasue of the distance 24 inchs from the tramsom first run was great and in a turn to port ok to stb it would suck air and cavitate so back to the shop and down 5 mm and back to the water and stb in a tight turn was perfect . The height we were able to jack the motor made a completely differant boat out of it when trimmed out . easy to steer . tracking dead straight and on the plane much quicker out of the hole !!.
My own boat has the 115 vee 4 yamaha 25 inch shaft set back 450 mm on a special purpose built transom i made and i set the height initally at 10mm above the hull bottom after the first run i was able to take it up another 20 mm comfortably so its now 30 mm above the hull bottom .
The bracket has made the power trim more sensitive than normal and a breeze to trim in any sea condition ,the steering is one finger at any speed and out of a hole its like a jet ski and jumps out of the water !! The boat is only 14 feet long and is a semi cabin built in 1975 !!It had a tired old 70 hp omc that i sold to some guy !!.
So all in all set up is very important not only for the motor but for the driver as well ,it becomes a pleasure to look forward to to drive and not a chore and hard work having to fight with everything !!!!

The wave hopping things is never a problem as your boat should be slightly tail heavy so shift a little weight till you get the trim where you want it and feels comfortable overreving. you hang on to the throttle and as you get to leave the water back off a little ita also helps to drag the nose down quicker and the open up and away again so you will need seat time to get the feel of what your new found friend likes and how to get the best from it !!. If you have to fight with it "YOU" are doing something wrong !! and "YOU " need to find out what and correct it !!