no more porpoising

Designs for inboard or outboard power

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BruceDow
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Re: no more porpoising

Post by BruceDow » Tue Oct 12, 2010 5:11 pm

Again, Dave... thanks for exploring this for us.

I recall that your porpising (like mine) was sensitive to the passenger load. (For me, three in the front and no-one in the back was my best performance at high speed)

Do you find that with the shims the performance is smooth across a variety of passenger loads? Or does it still vary with different weight distributions?

Thx.
B.
Bruce.

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DaveLott
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Re: no more porpoising

Post by DaveLott » Tue Oct 12, 2010 5:43 pm

The only passenger I had was my wife and I had her move around from seat to seat and to the back. There was no affect to the performance.

But I do believe I am a little aggressive with my angle at this moment. I am going to reduce down until I feel she is just balance right for these conditions:

2 people in front= 400#
fuel tank of fuel on transom = 170#
swim platform hanging off transom= 70#

and going 50mph.

I want to bring the angle down so that I can get to this speed under these conditions without any porposing in different types of seas.

I will have a full load in the rear compartment next week when I go on a fall color cruise about 80 miles up the lake. I will have the angle down to 3/8" and see what happens and report back to you
Dave

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Bill Edmundson
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Re: no more porpoising

Post by Bill Edmundson » Tue Oct 12, 2010 6:10 pm

Dave

I'd love to see a new picture at speed. I want to see the boat attitude compared with the before pictures! I think, I'm a little bow down. But, I don't bounce.

Even with a full load you were beaking the water about 18" to 24" behind the helm seat. Most "classic" boats seam to break the water right under the seat. I think mine is 8" to 12" forward.

This not from knowledge. Just looking at the boats.

Bill
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DaveLott
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Re: no more porpoising

Post by DaveLott » Tue Oct 12, 2010 7:24 pm

Bill Edmundson wrote:Dave

I'd love to see a new picture at speed. I want to see the boat attitude compared with the before pictures! I think, I'm a little bow down. But, I don't bounce.

Even with a full load you were beaking the water about 18" to 24" behind the helm seat. Most "classic" boats seam to break the water right under the seat. I think mine is 8" to 12" forward.

This not from knowledge. Just looking at the boats.

Bill

Agreed. Right now I am breaking just forrward of helm seat. I am riding low. But definitely was riding high
Dave

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Re: no more porpoising

Post by Oyster » Wed Oct 13, 2010 3:53 pm

http://www.grand-chronicle.com/

page 9
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upspirate

Re: no more porpoising

Post by upspirate » Wed Oct 13, 2010 3:55 pm

I see there was another boat built from a "kit" there!!!!

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DaveLott
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Re: no more porpoising

Post by DaveLott » Wed Oct 13, 2010 4:39 pm

upspirate wrote:I see there was another boat built from a "kit" there!!!!

That boat was ACTUALLY built from a kit. It is a James Craft.

I was the only handcrafted boat at the show.

The boat next to mine (the Hackercraft Gentlemen's racer) won the Contemporary award.
Dave

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Re: no more porpoising

Post by sschefer » Thu Oct 14, 2010 5:18 pm

I realize this is the most cost effective solution for you but have you thought about prop styles. A prop with better stern lift often overcomes bowlifting without need to add hook at the stern. The reason I mention it now is because you've found the problem and to that there is more than one solution.
Steve Schefer
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Re: no more porpoising

Post by BruceDow » Thu Oct 14, 2010 6:01 pm

Thanks for adding that.

I have also been porpoising at my high end. I have not yet added shims. I have been investigating "prop rake" to see if that would make a difference. My prop appears to have a slight positive rake. I have been advised that a neutral or negative rake should reduce bow lift/increase stern lift.

I guess I need to talk to a "prop shop" expert to see if we can try some out and see if they make a difference.
Bruce.

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~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
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Re: no more porpoising

Post by DaveLott » Fri Oct 15, 2010 5:50 am

Interesting point. I had planned to go to our local prop shop next week regarding prop sing. Above 32mph, the prop sings a very merry tune until about 40. Since I like to cruise in the mid thirties I get the consistent ringing in my ears.

We will report back on this. This shop is regionnally known as the best so we will find out

dave
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Re: no more porpoising

Post by GLH » Fri Oct 15, 2010 6:20 am

Dave, here is a shot of the gentlemans racer that was next to your boat. I think this is the one you are talking about???
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Re: no more porpoising

Post by PeterG » Fri Oct 15, 2010 11:46 am

Dave, I read earlier about your prop singing and I wonder if your running a bit too much blade loading? Check out David Gerr's propeller handbook and you'll see that a prop with a bit too much loading could suffer this singing effect. Ask about a prop with a higher Disk Area Ratio, which essentially means same diameter and pitch but each blade has more surface area with a stronger blade root at the hub, and a reduced tendency to vibrate. Also, check out Michigan Wheel's web site where they give a good description of propeller characteristics.
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Re: no more porpoising

Post by garrys » Fri Oct 15, 2010 12:05 pm

Dave, did you lay a straightedge on the bottom of your boat, fore and aft? Did the straightedge indicate anything other than perfectly straight (flat)? I remember the building instructions for my Monaco emphasizing that the bottom of the boat be perfectly straight for the last 5 feet or so, and also that the transition of the bottom to the transom be as "sharp" an angle as possible. A rounded corner will cause porposing. I don't have that problem with my boat, but then the only storage available on the Monaco is under the front deck, where I keep my anchor, chain, mooring lines, life vests, fire extinguisher, fender bouys, and beer cooler, so I've got quite a bit of weight up there to keep the nose down. Interestingly, I was at a water ski tournament this past weekend where I saw a 20' outboard with twin 300HP engines. The owner told me he had to put 300 pounds of lead in the nose to keep the boat from doing wheelies!

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Re: no more porpoising

Post by DaveLott » Fri Oct 15, 2010 7:18 pm

Garry. My bottom is true and the transom edge sharp. The alignment of the keel, strut and rudder are dead on. Everything is the way she supposed to be. I also thought about some sand bags up front as well.

But this is working so well, I am just going to dial it in with the correct size and leave it alone.


Yes, I have heard about some of those guys talk about getting weight way up front. That is maybe why I wanted to use Warren as a hood ornament.

dave
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Re: no more porpoising

Post by DaveLott » Fri Oct 15, 2010 7:19 pm

GLH wrote:Dave, here is a shot of the gentlemans racer that was next to your boat. I think this is the one you are talking about???

Yes, Gary, that is the one that won the best contemporary award. I believe it was built in 98 by Hacker Company.
Dave

Riviera build - the Midnight Cry Project
Glen-L Sea Kayak
Mahalo Standup Paddleboard

Video of Midnight in Action

Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

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