Rudder design

[nickyp]

A mishmash of questions for you all:

What does the amount of rudder blade in front of the pivot point have to do with performance/turning? What would a rudder with virtually no blade in front of the pivot act like? Please bear in mind, my boat is slow. Maybe 10 knots wot.
The reason I ask, is that my rudder an prop are awfully close, and i think it is causing turbulence. I can hear cavitation at some rpms, and don't know weather it is coming from the prop hull clearance, the back of the keel, the prop/rudder or all of the above.
I made my rudder out of foam and glass, in the typical fiberglass rudder shape.
And like I said, it is very close to the prop. From reading earlier threads, perhaps a thin metal one may be better?

I was on Michigan wheels web site. I have a brand new 14 square prop. I don't know the model off hand, but it is a vey basic 3 blade. I get 3600 rpm on my little 3gm30 yanmar with this prop. I think I've got just over an inch of hull clearance. Their Djx prop is said to work in situations of limited prop clearance. How and why?
I am going to try to get my transducer and gps installed this weekend, and jump overboard to scrub my hardware and bottom and take measurements as well

I know this is a garbled bunch of questions, but any insight would help

 

[South Shore Boatworks]

rudder

A couple of thoughts on your caviatation. A balanced rudder has approx. 20% of the blade forward of the shaft. Unless you have a wide, blunt forward edge of the rudder, I would suspect that the clearance between the bottom of hull and the wheel is more of the issue. An inch is very close. To get a better explanation and solution, give Bob Turcotte, a naval architect that is on the forum a shout. His e-mail is [email protected]

 

[nickyp]

A couple of thoughts on your caviatation. A balanced rudder has approx. 20% of the blade forward of the shaft. Unless you have a wide, blunt forward edge of the rudder, I would suspect that the clearance between the bottom of hull and the wheel is more of the issue. An inch is very close. To get a better explanation and solution, give Bob Turcotte, a naval architect that is on the forum a shout. His e-mail is [email protected]

Thanks, I will contact him
I will wait till this weekend, when I have some hard numbersto give

 

[nickyp]

Thanks, I will contact him
I will wait till this weekend, when I have some hard numbersto give

Another thought is this:
Which is the lesser of two evils, a bigger prop with less clearance, or a smaller prop with mor pitch.
I only have a 7/8 shaft, and obviously 14 is really pushing it , or exceeding it in the clearance dept.

 

[South Shore Boatworks]

rudder

It would be helpful if you have pictures of the running gear. Are there other boats like yours and if so how is their running gear set up?

 

[nickyp]

It would be helpful if you have pictures of the running gear. Are there other boats like yours and if so how is their running gear set up?

The boat is a 20 ft dyer, built in 1957.
They didn't make many of them.
I'll have to haul it and take some pics

 

[South Shore Boatworks]

dyer

I am familiar with your dyer. I now understand the size of your engine and running gear. My question is did something change that has increased the cavitation? did you repower, change the wheel, install that new rudder, is the wheel fouled or bent? These are some of the questions that a good prop guy will ask. Anchorage Corporation of Warren Rhode Island, the builder of your boat is still in business. You might give them a call

 

[nickyp]

I am familiar with your dyer. I now understand the size of your engine and running gear. My question is did something change that has increased the cavitation? did you repower, change the wheel, install that new rudder, is the wheel fouled or bent? These are some of the questions that a good prop guy will ask. Anchorage Corporation of Warren Rhode Island, the builder of your boat is still in business. You might give them a call

I was given the boat as a wreck, and spent 5 years of weekends gutting the hull to a bare shell, and rebuilding absolutely everything. You name it, and I made a new one.
The yanmar that came with it was in storage, along with an old milk crate of misc parts, including a very small damaged rudder, and a damaged 14" prop.
So I made a new bigger fiberglass rudder, and bought a new 14 prop.
Placement of the running gear is the same, except the new rudder is closer to the hull, and deeper than the old one.
I did contact dyer during the rebuild, but any questions I asked were answered that any info on the old 20s were lost in a fire years ago. They didn't seem too enthusiastic, so I never called back or sent them pics of the rebuild. Sad, really.
The yanmar was installed in 1980, to replace an old atomic 4 I was told by a local mechanic.
I think I just have to figure it out by trial and error, and taking any advice I can get along the way
Thanks for your interest

 

[traditions]

I don't think the rudder would cause avy vibration.I would get rid of the 3 blade,and try a 4 blade.Youcould use a smaller wheel and it would be way smoother.3 blades don't go well with a stern post.On a strut they may be fine,but not on a full keel.Is the problem all the thru the rpm,or just on the top end? If its wheel slap you can hear it on the side that the rotation is towards,more than the other side.I had a boat that had a lot of wheel noise and I did a lot of stuff to rid it,and the best thing was a dqx style wheel.

 

[Brooksie]

To answer one of your original questions, a rudder with no balance (no blade in front of the stock) will take more force to steer but will have no other ill effects. 20% ahead of the stock is about maximum beyond that it is hard to bring her out of a turn at speed.

 

[nickyp]

Traditions: I couldn't find a 4 blade small enough. I am running a 14 inch right now, and haven't seen a 4 blade under17.
The shaft goes through a strut. The whole setup looks exactly like a fortier 26 setup.
The cavitation seems to be only in the mid rpm range. Low and wide open seen ok.

Brooksie: I was given a big old steering ram that came off a Cabo, and that's what is in my little 20 fter. So it's good to know that I could use an unbalanced rudder and get good steering, and more clearance from the prop,while the ram would have no problem pushing it around. Especially at my speed. Thanks.

 

[Toolate]

If you did make a new rudder, would you have it made in bronze, stainless or go glass again?

 

[nickyp]

If you did make a new rudder, would you have it made in bronze, stainless or go glass again?

If a smaller prop doesn't give me satisfaction, and I decide to try a more unbalanced rudder, I think I would price out the local stainless fabricator to do me one up.
I've read the past discussions on rudders here and a lot of people are in favour of the reduced drag of a thin metal one as compared to the chunkier glass ones.
Don't know if if will make a whole hell of a difference, considering the low hp I'm working with. But I'd still be interested in finding out!

 

[Toolate]

I actually spoke to Jamie Lowell about this for my Sisu which has the fattest rudder you have ever seen and his suggestion was to maintain a wing shape (in cross section- round in front and tapering toward the rear) no matter what to avoid vibrations.

Seems like common sense I guess but it makes me think twice about a flat plate with bar stock welded to it but there are so many around.

 

[nickyp]

That is good to know. Because when ii made my fiberglass one, I did a lot of research as to the dimensions and shape. It is a wing shape, and the boat turns ridiculously tight at low speeds. I'm hoping the rudder is fine, and it is a prop clearance issue