Foils

FYI it's ventilation not cavitation you are discussing here. Also called aeration.

 

Thats all valuable info, thanks.Seeing as the boat is still being renovated and re-designed somewhat,it gives me an insight into the finished project.
My boat is only a foot shorter than yours,but I wouldnt like to imagine what it would do with your 115hp V4 Yammy on it.
I have to trailer the boat 70mls to the sea,so fine adjustments past a point have to be done once it is back home.No doubt I will come up with some ingenious plan to do it onsite.Haha.
I am moving the covered bow area forward about 6ins and fitting a lightweight cuddy,so no doubt this will counteract the tail heavy somewhat,and improve the trim.
Once again,much obliged for the info

 

Thanks Tom,I'll get these boating terms right eventually,its all being compiled.
And as Grannie said when she urinated in the ocean "Every little helps"

 

At the time of rebuild i was searching for a 3 cylinder 90 yamaha and had one lined up but time come to get it it was gone so took the next best thing 115 , I specially wanted a 25 inch shaft so i could fill in and glass over all the holes in the back of the boat . It never get water in from outside just when i wash the boat and flush all around under the floor with fresh water , The boat has a completely differant smell to what it used to have plus when it dries out its 100% dry all the time .
The 115 is hardly working and at 3500 when you push the throttle lever down you have to hang on because there no hessitation it jumps away . I was given a 21 inch prop to replace the 18 it had . slower in the lower rpms than the bigger diameter smaller pitch but the 21 with smaller diameter at 2500 is just starting to become evil . 3000 is comfortable and cruisie max at 6200 on the counter and off the clock !!is doing 60 plus mph, have to watch the head winds, can feel the bow starting to lift . i love it !!,its better than sex !!

 

Sounds like you have found a quick way to Heaven,lets hope not too soon
I remember seeing a boat in melbourne Australia in the 60's,it had twin merlin engines in it[Rolls Royce spitfire engines]It almost stood on end when he opened it up,I've never seen anything like it.

 

Honest power !

The good thing about having lots a power on tap is you dont need to use it !!
The motor will last longer , its pretty economical to run , its not slogging its guts out ever and if its a beautiful day and the water is like a mirror then you can let you hair down and go for it !!, Speed over water is the next best thing to flying and sex dosent even come close !!
When you are cruising and some one is passing you with a cheeky smile you can match his speed untill he hasnt any more to give then just gently pull away and know you did it honestly and with very little effort !!

I always remember a sticker on the back window of a car i saw ,it said "never drive faster than angels can fly"

 

I've installed DoelFins and several similar ventilation plate foils, all with good results. They are only good for getting a boat up, so in affect are intended for under powered boats that need a helping hand out of the hole. At high speed they are a huge determent and shouldn't be employed on craft seeing more then 30 knots.

A classic example is the last set I installed on a fairly heavy pontoon boat. It was struggling to get up and remain on plane. It had just enough HP to get up, but could get knocked off with moderate chop. The fins improved it's hole shot sufficiently enough to get it up at a lower speed and hold it there, when wakes and chop slapped it around.

As previously mentioned, at the other end of the spectrum these would cause considerable drag and likely substantial handling issues.

 

Thanks PAR,I dont think my boat will do much above 20 knots,but it sounds like it could have the torque and planing I am looking for.
You advised me recently on the width of the gap in the transom I would need for the outboard,unfortunately I didnt make a note of it,could you advise me again please.I am thinking of fitting a splash tray on the transom,
do you think it is possible with the boat having tiller steering or would it be in the way?

 

I know the feeling of having the extra power from years ago when I rode a motorbike,I might get a look off some body next to me at the traffic lights,the next time I saw them it was in my mirrors,it's a lovely concieted feeling,I could never suppress a grin.
I'm getting near my "Sellby date"now,so fishing has become my forte'

 

Remind me what engine/boat/HP again . . .

A moderate speed boat can easily benefit from a DoelFin or other ventilation plate wing. You boat sound like it might need one.

 

Hi PAR,Its a 1960's 13ft f/glass speedboat,with a 1964 Evinrude 18hp fast twin longshaft outboard.You recomended I changed the transom from a short to a long.Its light in weight enough for 4 guys to lift it onto the trailer.

 

valvebounce - what is the design/manufacturer of the boat that you are re-furbing? what problem are you trying to solve with the 'whale fins'?

 

Hi JB,I cant ID the boat,the hull is almost flat from 2/3 the way down,and 13ft long.Par recomended I use a 25hp outboard max.and to use it inshore only.ie bays etc.
I have not had the boat in the water yet,so what I am trying to achieve is the best way to go about the refurb.I am intending using the boat for a bit of fishing,so speed is not too important,but planing at lower speeds and economy is what i'm after.I thought Doel fins might help.
There seems to be a difference between Whale tails and Doel fins,the latter having a fin either side of the cavitation plate,where the the whale tail is in one piece and covers above the prop.Once againI would like to know the best to use,if any.
Best regards V

 

Both of these ventilation plate foils work the same way. Some are designed for higher speeds and typically use a delta shape, while the lower speed versions look like stubby aircraft wings.

The logical thing would be to launch and try her out. The 18 may be just what you want, though the DoelFin will help you stay on plane at lower speeds.

 

V - if you can send me a picture (email or PM) of your boat or post photo here, i will try to help. the effects of the fins will depend on the hull design and setup. They can help with getting on-plane, but can affect handling and performance at higher speeds. will help if i can see pic of boat.