no more porpoising

Designs for inboard or outboard power

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DaveLott
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no more porpoising

Post by DaveLott » Sun Oct 10, 2010 2:47 pm

Ok, I finally got rid of the porpoising. The boat has been real finicky at high speeds. I can get her up to 60 at times and then other times not over 40 without severe porpoising. Any little roll with speeds over 45 sends the boat into strong porpoising. So Midnight was right on the edge of being balanced for high speed. John and Gayle got one of the few rides over 50mph without the porpoising.

I did some studying. Some reports I read as to why the porpoising happens are rather interesting. They state essentially, if the boat is at too high of angle of attack (even a couple of degrees) the boat will begin to push a wall of water in front. It will reach a speed where it wants to try to climb up and over the wall causing the bow to drop. When the bow drops, it rebounds from the impact springing it back into the air allowing a larger wall to be quickly rebuilt. And the cycle continues to feed on itself. Up and down in higher and higher amplitudes.

So I took some advise and installed (2) 6x2 1/2" shims tapered from 1/2" to nothing. This was to force the bow down and decrease the angle of attack. They are mounted exactly one foot inside the outside edge of the chine and 1/8" inward of the transom edge.

Midnight is fast and smooth on acceleration and flat anyway but I noted an immediate effect. We hit 40 mph in rough water like NOW and it was smooth as silk. I pushed her all the way thru waves, on flat water, angling and straight on other wakes all over 40 and she just laughed it off. Smooth as silk and solid. You could literally feel the difference. Before Midnight felt very light and scary as if she could go airborne. Now she sits tight and solid. I can feel her weight on the water. I can feel her bow being pushed down near 50mph so I know the shims are a little big.

So now I am going to start cutting the length down until I get some porpoising then I will know where to leave it.

I share this only for the benefit of others that are having porpoising issues. The solution is rather quick and easy.
Last edited by DaveLott on Sun Oct 10, 2010 3:14 pm, edited 1 time in total.
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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BruceDow
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Re: no more porpoising

Post by BruceDow » Sun Oct 10, 2010 3:05 pm

Dave.

Thanks for pathfinding for us on this one.

I had planned to do that, but have been distracted with some other priorities.

Do you have pics? Did you screw them on or just epoxy them on?

is 6" the length or the width of the shims?
Bruce.

~~ Do what you love, and love what you do. ~~
~~ 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) ~~
Dow's Monaco Project

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kens
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Re: no more porpoising

Post by kens » Sun Oct 10, 2010 3:09 pm

So, bascally your saying you added a little hook on the running bottom to push the bow down, correct?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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DaveLott
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Re: no more porpoising

Post by DaveLott » Sun Oct 10, 2010 3:12 pm

Bruce Dow wrote:Dave.

Thanks for pathfinding for us on this one.

I had planned to do that, but have been distracted with some other priorities.

Do you have pics? Did you screw them on or just epoxy them on?

is 6" the length or the width of the shims?

the 6" is the length. I am going to cut this down some more until I find the balance.

I saw a gentlemen's racer made by Hackercraft with 4" ones installed at the show last week.
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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DaveLott
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Re: no more porpoising

Post by DaveLott » Sun Oct 10, 2010 3:13 pm

kens wrote:So, bascally your saying you added a little hook on the running bottom to push the bow down, correct?

Yes, a small one localized. It has the same effect as trim tabs but mine is fixed. I did not want to hang those big aluminum tabs off the back so this is next best solution.
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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Dave Grason
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Re: no more porpoising

Post by Dave Grason » Sun Oct 10, 2010 4:39 pm

Wow, this is great to know. I remember at the G4, we took off up the river and we were discussing this. When you had Warren and TerryMc come from the rear cockpit to the front, I noticed some difference then. So you were on the right mental track. You just needed to follow through even more.

Kudos to you and thank you for sharing.
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

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DaveLott
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Re: no more porpoising

Post by DaveLott » Sun Oct 10, 2010 6:13 pm

Dave Grason wrote:Wow, this is great to know. I remember at the G4, we took off up the river and we were discussing this. When you had Warren and TerryMc come from the rear cockpit to the front, I noticed some difference then. So you were on the right mental track. You just needed to follow through even more.

Kudos to you and thank you for sharing.

Dave - When I had Warren and Terry move forward I expected better response and was stumped by the lack thereof. After some thought, here is my conclusion. The passengers in the front compartment on the bench seat are within inches of the COB. Therefore, the weight movement forward was nominal at best.

Now if I had made a hood ornament out of the Pirate, then I would have been really on to something. :lol:
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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DaveLott
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Re: no more porpoising

Post by DaveLott » Sun Oct 10, 2010 6:21 pm

Bruce Dow wrote:Dave.

Thanks for pathfinding for us on this one.

I had planned to do that, but have been distracted with some other priorities.

Do you have pics? Did you screw them on or just epoxy them on?

is 6" the length or the width of the shims?

Oh, forgot to answer you Bruce. I countsunk 2 #8 x1" SS screws and then applied thickened epoxy to the leading edge to prevent water forcing itself between the shim and the boat and ripping it off.. I thickened with wood flour. Once I have the length dialed in, I will encapsulated the white oak shim and then fully epoxy it down. I want to the transition from the hull bottom onto the shim to be smooth.
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.

Oyster
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Re: no more porpoising

Post by Oyster » Sun Oct 10, 2010 6:23 pm

Great to hear this Dave... 8)

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psychobilly
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Re: no more porpoising

Post by psychobilly » Mon Oct 11, 2010 1:58 am

I think I've posted this here before put my skiff has a natural "hook" in teh stern and she pushes her bow down if you get up over 30. But, Uncle Jeral built her that way as she was a werk'n boat. She can get 20 sacks of oysters on step, it ain't fast but you can get her on step and she rides GREAT. I'm not as fortunate as you Dave to be able to play with that fine line. Happy ta hear she's come'n around!!!

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Dave Grason
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Re: no more porpoising

Post by Dave Grason » Mon Oct 11, 2010 7:47 am

DaveLott wrote: Dave - When I had Warren and Terry move forward I expected better response and was stumped by the lack thereof.
You know, that makes sense. That was the first time I had ever ridden in your boat and so I didn't know exactly what you were expecting. But I gotta tell ya, these experiences are a very fun learning experience for all of us. If there is any one single thing about going to the Gatherings, it's the ability to actually get into the boats and "try them out" and at the same time being able to talk to the people that built those boats.

Wow, the value of these Gatherings cannot be underestimated.
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

ToneDef
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Re: no more porpoising

Post by ToneDef » Tue Oct 12, 2010 11:54 am

Good suggestion, glad to hear it worked for you.


One easy way to see if this will fix your porpoising issue is to tie a length of rope around the boat, in just the location you would be putting the wedge (i.e. across the bottom near the transom, up the sides, tied on top of the deck. This effectively serves the same purpose as the wedge, but allows easy removal if the problem is not solved.

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BruceDow
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Re: no more porpoising

Post by BruceDow » Tue Oct 12, 2010 12:59 pm

ToneDef wrote:Good suggestion, glad to hear it worked for you.


One easy way to see if this will fix your porpoising issue is to tie a length of rope around the boat, in just the location you would be putting the wedge (i.e. across the bottom near the transom, up the sides, tied on top of the deck. This effectively serves the same purpose as the wedge, but allows easy removal if the problem is not solved.
Now THAT is worth the price of admission!

I had heard that the "cheap and cheerful" way was to tack a strip of rubrail along the transom. But a rope is even more "cheap". I may give that a try next season, before committing to wedges.
Bruce.

~~ Do what you love, and love what you do. ~~
~~ 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) ~~
Dow's Monaco Project

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Iggy
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Re: no more porpoising

Post by Iggy » Tue Oct 12, 2010 1:33 pm

I am sure a lot of us would love to see a picture.. I am having a hard time visualizing these parts.
Ian (aka Iggy)
My Malahini Build

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DaveLott
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Re: no more porpoising

Post by DaveLott » Tue Oct 12, 2010 4:59 pm

This is all I did. I sanded the bottom paint down to the primer for a bonding surface and then moved the pad 3" outboard. When I settle on the final angle I will remove, sand off bottom paint, and epoxy everything together

Decided to grind down the height from 1/2" to 3/8" and see how that feels. If I still feel the nose is digging too much at high speed, I will go down to 1/4". I have funny suspicion, the 1/4" will be all that is needed but you just don't know until you start fiddling with it.
100_4657.JPG
Please understand this is not the best solution. The goal of any boat is to have the hull as level with the surface of the water as possible while keeping the bow up and having the least amount of hull surface on the water. That is why we have adjustable trims on outboards so that we can adjust on the fly. The optimal solution would be to install hydraulic trim tabs on the transom. However, I am not fond of those large tabs on the back of a woodie, so this is next best solution. Unfortunately, I know I will lose some top end as it will tend to force the bow down too much at high speeds but then again, how often will I really run with all bores to the wall?

And to add more substance to the discussion; we are only dealing with a 7 degree deadrise at the transom. This is virtually flat compared to the 22-24 degree deadrises we find on a large number of production boats now.
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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