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From Archive: Porpoising by DonJF

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From Archive: Porpoising by DonJF

Post by Club Archivist » Fri Jul 28, 2006 7:21 pm

I apologize for bringing up this topic again as it was discussed back in August 2000 I just need a follow-up on the problem of pospoising of a '63 CC ski boat. I got my boat in the water three times last summer and had a terrible issue of porpoising and must address it now.

Comments were made by Don Ayers that using two 6" X 6" wedges tapered to
1/2" placed approximately 14" on each side of the rudder greatly reduces
this problem on a 1949 18' CC. Ray Barber also commented.(where are you
Ray?) I'd greatly appreciate any additional comments.

DonJF
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From Archive: Porpoising by DonJF

Post by Club Archivist » Fri Jul 28, 2006 7:26 pm

Don;

Porpoising can be a problem with many of these smaller boats for reasons Ray
pointed out. It think that two things can make the biggest difference.
  1. 1. The center of gravity is to far aft. This is very similar to the CG of
    aircraft. The CG affects the attitude at which the boat will ride and the
    planing surface location. I once saw a 17' Barrel back that a guy totally
    reworked. He decked over the front cockpit making it look like some kind of
    gentlemen's racer and put a 350 SB in the engine compartment. The boat
    porpoised so badly that he could not even get up on plane. It was a sad
    sight for a once beautiful 17' Barrel.
    2. The planing surface and aft is not fair. If there is hogging and
    warping of the planing surface this acts like a wedge and puts down forces
    or the opposite on the attitude of the boat.
In the case of the 1949 18' Sportsman there were several factors to
consider.
  1. 1. The reconstructed boat was lighter than the original due to a light,
    strong multi-lamination bottom.
    2. The owner wanted to replace the KL 105 HP with a strong V8 (325 HP).
    3. The hull was not designed to go that fast with that particular CG even
    though the KL and V8 were similar in weight.
So we were close to the original CG but with the loss of weight and the
higher speed we developed a porpoise above 40 MPH. It was not dangerous
just annoying to the owner because he liked to get out and run her up on a
smooth day. During test trials with no interior in the boat and two in the
front the boat did not porpoise at speeds up to 50 MPH. Once the boat was
completely assembled and with a tank of gas the porpoise showed it's ugly
head.

I used a 6" X 6" X 1" block of Lexan (Clear) and cut it on a diagonal to
produce a 1/2" rise. I used epoxy to glue to the bottom in the location I
mentioned. The cool thing was the wedge was clear so it literally
disappeared and it does not catch your eye. The owner then added 100 LBS of
weight to the nose in the form of two 50 LB free weights and no more
porpoise.
She tops out at just under 50 MPH because of the prop I used. I did not
want him going any faster than that. I used a lower pitch so it will come
out of the hole like a comp ski boat.

Thx

Don
Posted by Club Archivist
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Club Archivist
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Posts: 164
Joined: Fri Nov 11, 2005 9:15 am
Location: Club Headquarters

From Archive: Porpoising by DonJF

Post by Club Archivist » Fri Jul 28, 2006 7:26 pm

Don;

Porpoising can be a problem with many of these smaller boats for reasons Ray
pointed out. It think that two things can make the biggest difference.
  1. 1. The center of gravity is to far aft. This is very similar to the CG of
    aircraft. The CG affects the attitude at which the boat will ride and the
    planing surface location. I once saw a 17' Barrel back that a guy totally
    reworked. He decked over the front cockpit making it look like some kind of
    gentlemen's racer and put a 350 SB in the engine compartment. The boat
    porpoised so badly that he could not even get up on plane. It was a sad
    sight for a once beautiful 17' Barrel.
    2. The planing surface and aft is not fair. If there is hogging and
    warping of the planing surface this acts like a wedge and puts down forces
    or the opposite on the attitude of the boat.
In the case of the 1949 18' Sportsman there were several factors to
consider.
  1. 1. The reconstructed boat was lighter than the original due to a light,
    strong multi-lamination bottom.
    2. The owner wanted to replace the KL 105 HP with a strong V8 (325 HP).
    3. The hull was not designed to go that fast with that particular CG even
    though the KL and V8 were similar in weight.
So we were close to the original CG but with the loss of weight and the
higher speed we developed a porpoise above 40 MPH. It was not dangerous
just annoying to the owner because he liked to get out and run her up on a
smooth day. During test trials with no interior in the boat and two in the
front the boat did not porpoise at speeds up to 50 MPH. Once the boat was
completely assembled and with a tank of gas the porpoise showed it's ugly
head.

I used a 6" X 6" X 1" block of Lexan (Clear) and cut it on a diagonal to
produce a 1/2" rise. I used epoxy to glue to the bottom in the location I
mentioned. The cool thing was the wedge was clear so it literally
disappeared and it does not catch your eye. The owner then added 100 LBS of
weight to the nose in the form of two 50 LB free weights and no more
porpoise.
She tops out at just under 50 MPH because of the prop I used. I did not
want him going any faster than that. I used a lower pitch so it will come
out of the hole like a comp ski boat.

Thx

Don
Posted by Club Archivist
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This topic was reposted from Memberclicks archive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Club Archivist
Site Admin
Posts: 164
Joined: Fri Nov 11, 2005 9:15 am
Location: Club Headquarters

From Archive: Porpoising by DonJF

Post by Club Archivist » Fri Jul 28, 2006 7:27 pm

Don;

Porpoising can be a problem with many of these smaller boats for reasons Ray
pointed out. It think that two things can make the biggest difference.
  1. 1. The center of gravity is to far aft. This is very similar to the CG of
    aircraft. The CG affects the attitude at which the boat will ride and the
    planing surface location. I once saw a 17' Barrel back that a guy totally
    reworked. He decked over the front cockpit making it look like some kind of
    gentlemen's racer and put a 350 SB in the engine compartment. The boat
    porpoised so badly that he could not even get up on plane. It was a sad
    sight for a once beautiful 17' Barrel.
    2. The planing surface and aft is not fair. If there is hogging and
    warping of the planing surface this acts like a wedge and puts down forces
    or the opposite on the attitude of the boat.
In the case of the 1949 18' Sportsman there were several factors to
consider.
  1. 1. The reconstructed boat was lighter than the original due to a light,
    strong multi-lamination bottom.
    2. The owner wanted to replace the KL 105 HP with a strong V8 (325 HP).
    3. The hull was not designed to go that fast with that particular CG even
    though the KL and V8 were similar in weight.
So we were close to the original CG but with the loss of weight and the
higher speed we developed a porpoise above 40 MPH. It was not dangerous
just annoying to the owner because he liked to get out and run her up on a
smooth day. During test trials with no interior in the boat and two in the
front the boat did not porpoise at speeds up to 50 MPH. Once the boat was
completely assembled and with a tank of gas the porpoise showed it's ugly
head.

I used a 6" X 6" X 1" block of Lexan (Clear) and cut it on a diagonal to
produce a 1/2" rise. I used epoxy to glue to the bottom in the location I
mentioned. The cool thing was the wedge was clear so it literally
disappeared and it does not catch your eye. The owner then added 100 LBS of
weight to the nose in the form of two 50 LB free weights and no more
porpoise.
She tops out at just under 50 MPH because of the prop I used. I did not
want him going any faster than that. I used a lower pitch so it will come
out of the hole like a comp ski boat.

Thx

Don
Posted by Club Archivist
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This topic was reposted from Memberclicks archive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

THE RAZZ
Posts: 583
Joined: Thu Apr 20, 2006 9:52 am
Location: OAKLAND, CA

re porposing

Post by THE RAZZ » Fri Jul 28, 2006 11:34 pm

DonJF
I agree with Don Ayres, as an A&P mechanic, the center of gravity (CG) and various methods to move the CG including the use of hidden trim tabs (6"X6") wedges will affect hull pitch oscillating -the porpoise up to a point.

However, something was left out of the equation. Principally (but not exclusively) the hull length (prop shaft angle) determines maximum speed and the onset of the hull pitch oscillation.

17' , 19' or 23" boats - for example - each have less shaft angle as the hull length increases. Consequently, the longer boats are capable of proportionally greater speeds up to a point.

Shifting the boats CG with different methods will attenuate the oscillation but not eliminate the force causing the changes of pitch. So, applying more power (i.e.350 small blocks) to the shorter boat's greater shaft angle simply forces the back of the boat up and the bow down in an oscillating change of pitch - sooner. In contrast, increasing power of a longer hull with shallower shaft angle will delay the critical point where the longer boat ultimately begins to oscillate.

Some modern high power small runabouts avoid the shaft angle pitch oscillations by using rear engine stern drives that effectively eliminate the shaft angle. Hydraulic trim tabs, of course, adjust changes in CG. Earlier, of course, V drives allowed weight to be shifted further aft while simultaneously reducing shaft angle dramatically increasing speeds for high powered small boat delaying the onset of pitching oscillations.

The 2006 Shelby 22' Donzi, for example, with almost 500 hp driving nearly straight out of the stern drive is capable of nearly 85-mph

Note in the attached picture where the hull separates from the water at the rear of the aft cockpit near the front of the engine. The hull is near its critical speed and will begin a slow oscillation that increases dramatically if the hull is pushed beyond 38 mph. Some attenuation of the pitching oscillations may occur with the 6x6 wedges but to drive this 17' hull much beyond 40 mph is problematical. Some argue the cost of speed is the square of the increase. Look at the new 38' Donzi stepped hull for $350K+. Yikes.

Good luck, JerryT Image

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