Propeller on BR Spray 46

[almadelmar]

Hello everybody! I have a question along the shaft line and the propeller of the boat Spray 46 from the Bruce Roberts. I build it, and it seems to me, that the propeller is too close to the waterline. Are there any owners of such boats? When boat on the wave, can propeller come out of the water? Maybe it's worth changing the design and drowning the propeller? Thanks to all!

[Jim Cate]

I can not help you directly, but in your position, I'd contact the folks who designed the boat: Bruce Roberts design.

I've seen a few of these boats floating around, and not noticed that their props were unusually close to the LWL, but I was not looking critically at that feature.

Jim

[almadelmar]

Thanks, Jim. I want to read answer of owner, I think it would be better, than answer of designer ))

[boden36]

What makes you think it is too shallow?
Has the boat been built following the plans?
Is the boat in the water and not working properly?
How far below the WL is the top of the prop?
If you are still building, be aware that on launching she may float below her marks.

Regards,

Richard.

[almadelmar]

Boat is under construction now. The top of prop is nearly 30 cm below of theoretical WL. And, yes, i build it following the plans.

[Jim Cate]

My observation is that most home builds float a bit below their plan LWL! Plus, most boats squat a bit under power, so I suspect that you will be ok.

Jim

[darylat8750]

Some boats actually have the top of their props above the waterline. Don't worry about it. Build it to plans. Changes always have unintended consequences ...

[boden36]

Build it to the plans. We built our steel yacht, and almost every change that I made to the original plans had unintended consequences.
Who are we, with our limited knowledge, to second-guess an experienced naval architect on any basic design parameter?

Regards,
Richard.

[IdoraKeeper]

My Ingrid is afflicted but the prop being too close to the water line. Under certain conditions the prop will breech the surface. Cavitation can also occur. Prop placement was driven by the location of the engine logs. It's all about the geometry. You may not have a choice.

[Jim Cate]

Originally Posted by boden36

Build it to the plans. We built our steel yacht, and almost every change that I made to the original plans had unintended consequences.
Who are we, with our limited knowledge, to second-guess an experienced naval architect on any basic design parameter?

Regards,
Richard.

This is wisdom! Over the years I've heard and read comments from designers who bemoaned the "improvements" amateur builders have introduced, never yet heard one say "now why didn't I think of that"!

Jim

[Fore and Aft]

I surveyed a steel 40 foot spray in Bundaberg two weeks ago and the prop seemed in the right posistion. There must be hundreds of those steel sprays around and surely Bruce would have changed the design if it was an issue? Thinking about it and not trying to be rude, but triple check your measurements. If what you say is correct then the engine posistion must be well above the waterline? That spray I surveyed had the cockpit above the motor and just a slight angle on the shaft.
I just measured the tip of the prop to the LWL on a 36 steel spray on the slip here and it is 550mm.
My own 32 foot yacht only has 300mm from LWL to the tip of the prop and it seems to work fine.
As for trying to drown the propeller. My dads 44 foot fishing trawler had its prop down real deep and I can clearly remember being thrown on our beams end and that prop cavatating like crazy.
Cheers