Cavitating Ventilating Prop pictures added

[BilgeBoy]

Hi all

Quick update on where we are at....

Finally got a chance to have a play on the weekend and took a run over to Coochie. One the run over I got the missus to steer while I stuck my head over the back...the first thing I noticed was all the spray going everywhere....what a mess. Looked to be coming from the sacrificial anode area just above the cave plate and at 20Mph the foil is about 3/4 inch below the water line. Tried to trim in on turns and this was marginally better!!

So park her up on th beach...and whip the foil off. Load the crew on so we have the same conditions and go for a squirt....What a pig. The nose heads for the sky and is real slow to come back down. Onto the electric trim and in as far as I can go....the nose slowly comes down....but not as far down as with the foil. In fact with the foil attached...I can push the nose in until I get wash from up past me (front steer). Without the foil...its all nose in the air and unstable in the rough stuff.

I haven't done anything with the prop as yet....Will soon enough (Saving for a sounder at the moment). In the meantime I will RAISE the engine a whole...and see how that goes with and without the foil.

Hey I figure people have done a lot of 2 fingered typing on this post....so the least I can do is be a Guinea pig, try different suggestions...and report back!!

BilgeBoy

[Homer_Jay]

Not real sure if any of this info will help but here goes.....

About ten years ago my old man bought a 4.5 mt tinny runnabout brand new with a 50 Yammy on her. When we got it home and took it for a run it would vcavitate the prop all the time even in a straight line getting onto the plane. Needless to say he was not happy so he took it back to get it fixed. After alot of arguing and trouble they finally agreed that it wasnt right and they couldnt fix it so then Alf Stessl got involved and in the end they got it to be allright but it would still cavitate in the turns at any speed. What they did was put on a cupped prop. The old man just put up with it. Then about 18 months ago he decided it was time for a new outboard. Being pretty happy with the Yammy he got another one but this time got a 60 hp. The first thing we noticed was that the prop was so much bigger in diameter from the one that was on the 50. We were wondering if we were going to have more problems and have to get the prop cupped. But we took it for a run and what a different boat. Now the boat wont even cavitate in the tightest of turns. He couldnt believe it was the same boat. All that we can put it down to is the bigger prop. So dont discard the prop issue too easily. I would go have a talk to a prop expert.

[Chimo]

Hi Bilge Boy

Just looked at the photos and read you last comment and it really looks to me that the motor is too low.

Added to that impression is your comment that the foil is 3/4 " below the water when your running at speed on the plane which also reinforces this opinion.

My experience with cavitation foils etc on many boats is that if motor height is satisfactory and foils are fitted
1 there is not too much water /spray being thrown around and
2 I have always been able to stick my fingers / hand between the top of the water surface and the foil when planing at speed

There is too much drag if the foil / motor is too low.

You want the foil to do its thing as you go from displacement speed to planning speed at which time, if you don't have trim tabs you would be trimming the motor up, lifting the foils even higher, and making your steering lighter.

The prop could also be part of the problem too.

Perhaps you need to lift the motor: its too low anyway and your observation re the foil makes that plain; and talk to a good prop person and or maybe borrow a few to try and see the difference props may make.

Good luck, you'll get there in the end.

Cheers

Chimo

[a_big_red_1]

B/B reading your last post, IMHO i would lift the motor 2 holes
i have just been going through all this with my boat
lifted 1 hole made a difference
lifted another hole and it is a completely different boat

Daz

[BilgeBoy]

Found this little Gem on the Solas Props Web site....basically a warning at the top of their stainless prop page...looks like I will be raising the engine a whole before buying a new prop!!

http://www.solas.com.au/shopdisplayp...=44&cat=Yamaha

Again will keep ya posted.

[BilgeBoy]

Smiles all round fella's

I raised the engine a hole and took her for a spin up the Logan today!!! Talk about different boat!!! Not only has the cavitation all but gone on turns...but the boat feels way more stable. I can now trim the bow in as well (without the foil)...oh...and there is no more spray going everywhere...just clean air.

Full throttle 5500 RPM about 35Mph...happy on 4000...She's Sweet man Sweet

Thanks to all those who voted "UP"...and not down (especially you Finga...you won the bet mate). I gotta tell ya I have had so much advice on this one... so called specialists who said..."If it's cavitating its gotta go down" and Boy racers who said "Ya need a 15 pitch Stainless". Oh well....pays to open ya ears and listen and then make informed choices.

Again...THANK YOU TO ALL who have contributed to this post! I owe ya one and hope that one day I can return the favour. Have a very Merry Christmas and a happy New Year...and safe boating over the holidays. Hope to see ya out there!

BilgeBoy

[Sea-Dog]

Hi Bilge Boy.

....................My experience with cavitation foils etc on many boats is that if motor height is satisfactory and foils are fitted
1 there is not too much water /spray being thrown around and
2 I have always been able to stick my fingers / hand between the top of the water surface and the foil when planing at speed

There is too much drag if the foil / motor is too low...................



Good luck, you'll get there in the end.

Cheers

Chimo

Just how many fingers do you have left after your experiments Chimo????


- Sea-Dog

[finga64]

Smiles all round fella's

I raised the engine a hole and took her for a spin up the Logan today!!! Talk about different boat!!! Not only has the cavitation all but gone on turns...but the boat feels way more stable. I can now trim the bow in as well (without the foil)...oh...and there is no more spray going everywhere...just clean air.

Full throttle 5500 RPM about 35Mph...happy on 4000...She's Sweet man Sweet

Thanks to all those who voted "UP"...and not down (especially you Finga...you won the bet mate). I gotta tell ya I have had so much advice on this one... so called specialists who said..."If it's cavitating its gotta go down" and Boy racers who said "Ya need a 15 pitch Stainless". Oh well....pays to open ya ears and listen and then make informed choices.

Again...THANK YOU TO ALL who have contributed to this post! I owe ya one and hope that one day I can return the favour. Have a very Merry Christmas and a happy New Year...and safe boating over the holidays. Hope to see ya out there!

BilgeBoy

Woohoo she's a goer (and no new prop)
Told you all the props OK
Another good result...beauty, now to the important things....rod in boat and off

[Chimo]

Yo Ho Ho Sea Puppy!

When you write (print) your Xmas list, ask Santa for the big foils you know the ones that stick out a foot or more on each side of the fin, skeg, propeller thingo at the back of the boat.

Then you can play at the back of the boat too

Merry Xmas to you too.

Cheers
Chimo

PS Aren't these smiley thingos great

[Sea-Dog]

Yo Ho Ho Sea Puppy!

When you write (print) your Xmas list, ask Santa for the big foils you know the ones that stick out a foot or more on each side of the fin, skeg, propeller thingo at the back of the boat.

Then you can play at the back of the boat too

Merry Xmas to you too.

Cheers
Chimo

PS Aren't these smiley thingos great

Just tie my leash to the ski hooks, and off we go

[blaze]

The idea of the foil is to skin on the top of the water (I dont like them), I would think to have any chance of that happening with what I am looking it needs to lift at least an inch, dont know if being so low its causing a tubulance around the prop.
cheers
blaze
ps have you tried with the foil off

my first answer to ya problem, glad it all worked out for you
cheers
blaze

[FNQCairns]

I just had a quick play with the prop calc and your numbers, you are doing ok, at around 10% slip with your prop at WOT, you could do better but it can get costly, my 30hp Johnson had a prop not much smaller than yours, interesting how Yamaha have a gear ratio on these engines that make the the prop spin around 25% faster at the same RPMs than my 30hp engine, hence the need for a small dia n pitch. I suspect your boat is about the biggest boat a person would want to fit the yam 50 to, by the numbers anyway.

Glad it's finally treating you well.

cheers fnq