MODIFICATION: Hull Bottom Change to Reduce Porpoising in 13-Foot

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  MODIFICATION: Hull Bottom Change to Reduce Porpoising in 13-Foot

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Author Topic:   MODIFICATION: Hull Bottom Change to Reduce Porpoising in 13-Foot

13redwhaler posted 04-11-2006 09:05 PM ET (US)
I have a 1971 13-foot Boston Whaler boat. Are there any bottom modifications to improve the never ending porpoising of this model? Are working trim tabs installed [in order to eliminate] the same problem? Mark.
Royboy posted 04-11-2006 09:25 PM ET (US)
First, a little housekeeping: turn off your cap lock when you post here so your words don't look like YELLING.
Next, do a search on porpoising and you'll find that it has nothing to do with the superbly designed bottom of your Boston Whaler boat and everything to do with the height and angle of your engine.
I have power tilt and trim on my 13 Sport and I find that if I trim the engine too far back my boat will porpoise, but if I trim it tighter to the transom it stops. Basically, I start with it trimmed all the way in and once on plane I trim it back just until I feel the boat pick up speed and the hull goes quiet. Any more leads to porpoising.
Roy
13redwhaler posted 04-11-2006 10:03 PM ET (US)
Sorry about the caps. I did not notice. Yes, I understand engine height, trim angles, trim buttons, trim tabs, and gunwale trim. I have a 40-HP Mercury with a manual jack plate with a 3-inch set back to get engine height up and [anti-ventilation] plate level with the running surface of the water as it leaves the transom. Have modifications to the hull bottom been tried to reduce porpoising? Have trim trim tabs been added to reduce porpoising? Any success? --Mark.
13sport posted 04-11-2006 10:04 PM ET (US)

This previous discussion may help.
http://continuouswave.com/ubb/Forum4/HTML/002920.html

Sport
Binkie posted 04-11-2006 10:11 PM ET (US)
I never personally checked but I thought the 13 footer had a built in hook to reduce porposing. Probably Tom Clark would know. If so the bottom could be straightened to produce more speed.
Royboy posted 04-12-2006 05:50 AM ET (US)
I think you answered your own question: "with a manual jack plate with a 3-inch set back to get engine height up and [anti-ventilation] plate level with the running surface of the water as it leaves the transom."
Your engine is too high. It's losing its bite on the water and dropping the bow starting the porpoising action. Having the anti-cav plate precicely level with the bottom of the boat isn't necessarily the very best place for it. The manual says plus or minus an inch as a general recommendation. Should be an easy matter for you to find the optimium running height of your engine.
Roy
jimh posted 04-12-2006 08:33 AM ET (US)
Porpoising, a tendency for the bow of the boat to oscillate up and down when the hull is planing, can occur in any boat, although some configurations make this occur over a wide range of conditions. Generally adjustment of the engine trim angle will permit the boat to remain on plane and stop the bow oscillations.
Weight distribution in the boat also affects porpoising.
What are:
--trim buttons?
--gunwale trim?
I am not familiar with those terms, and I would appreciate an explanation.
Tom W Clark posted 04-12-2006 10:38 AM ET (US)
The 13 footer is not known for porpoising. I have run a great number of these little boat over the years including my own 1976 Sport 13 Center Steer. The 13 footer will generally run flat and stable if set up correctly.
The 13 foot hull has a hook designed into it NOT to reduce porpoising, but to help get it on plane and stay there. It may well effectively help prevent porpoising but I suspect in this case 13redwhaler is fighting some other element of his/her set up.
If the cavitation plate is flush with the keel, that is good. This certainly is not the cause of the porpoising. Generally, having the motor set to low will aggravate porpoising, not having it too high. It could well be that 13redwhaler's motor could even be mounted higher given it is set back somewhat.
The usual cause of porpoising is excessive rear weight bias. The set back bracket will aggravate this condition. To test if rear weight bias is contributing to the problem, try temporarily moving weight forward and running the boat again. Note of the porpoising stops or is diminished.
The use of power trim on a 13 is of little value. The 13, because of its hooked bottom and flat running attitude cannot have its bow lifted much. Power trim can be used to "tuck in" the motor to help the boat get on plane but once there trimming out beyond the point where the motor is vertical will have little benefit.
Royboy posted 04-12-2006 01:45 PM ET (US)
I agree with everything Tom said except about power trim on a 13. I have it on mine and it makes a world of difference. I can raise the bow up a good foot or so and either cause, or get rid of, tendency to porpoise with trim alone. I alsways start with the engine trimmed all the way in, and once on plane trim the motor out until the hull goes quiet. It equates to a few mph increase in speed. This is a 2000 model, however.
Roy
hmbbuddy posted 04-15-2006 01:49 AM ET (US)
Try using a dolfin Or what you called "wing" in your propeller area. It is a good add-on and stabilizes your run.
project13 posted 04-17-2006 09:26 AM ET (US)
"The use of power trim on a 13 is of little value."
I also really have to disagree here also. On my 1978 13 the power trim also makes a world of difference, not only in terms of performance but also in the ability to keep the ride angle at a point that keeps the ride smooth and dry, in both a head and following sea.
I would say that when I am in the boat by myself, the ability to trim out the motor is good for at least 5 mph. When I run flat out, the boat will not porpoise when trimmed out, but at anything less than full tilt, I need to keep the motor trimmed down to prevent it.
I have considered a stingray/doelfin but would really like a consensus on it before I drill holes in my brand new $4K motor. Anyone else have any thoughts on it?
Royboy posted 04-17-2006 09:53 AM ET (US)
I installed a Stingray Jr on my 25hp Bigfoot to try and help my boat plane faster. It helped marginally, but the actual fix was a lower pitch prop. I never has a porpoising problem that I couldn't fix with trim, so if I had bought the new prop fisrt, I wouldn't have a fin.
Roy
Teak Oil posted 04-17-2006 05:01 PM ET (US)
Roy dont you have a 130 Sport? That is a completely different hull than Tom is referring to.
13redwhaler, your problem is likely due to a combination of too much weight aft, engine mounted too low/high, or too much trim.
Modification of the hull is not necessary and a waste of time and money. You should be able to correct this without spending a dime

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Author	Topic: MODIFICATION: Hull Bottom Change to Reduce Porpoising in 13-Foot
13redwhaler	posted 04-11-2006 09:05 PM ET (US) I have a 1971 13-foot Boston Whaler boat. Are there any bottom modifications to improve the never ending porpoising of this model? Are working trim tabs installed [in order to eliminate] the same problem? Mark.
Royboy	posted 04-11-2006 09:25 PM ET (US) First, a little housekeeping: turn off your cap lock when you post here so your words don't look like YELLING. Next, do a search on porpoising and you'll find that it has nothing to do with the superbly designed bottom of your Boston Whaler boat and everything to do with the height and angle of your engine. I have power tilt and trim on my 13 Sport and I find that if I trim the engine too far back my boat will porpoise, but if I trim it tighter to the transom it stops. Basically, I start with it trimmed all the way in and once on plane I trim it back just until I feel the boat pick up speed and the hull goes quiet. Any more leads to porpoising. Roy
13redwhaler	posted 04-11-2006 10:03 PM ET (US) Sorry about the caps. I did not notice. Yes, I understand engine height, trim angles, trim buttons, trim tabs, and gunwale trim. I have a 40-HP Mercury with a manual jack plate with a 3-inch set back to get engine height up and [anti-ventilation] plate level with the running surface of the water as it leaves the transom. Have modifications to the hull bottom been tried to reduce porpoising? Have trim trim tabs been added to reduce porpoising? Any success? --Mark.
13sport	posted 04-11-2006 10:04 PM ET (US) This previous discussion may help. http://continuouswave.com/ubb/Forum4/HTML/002920.html Sport
Binkie	posted 04-11-2006 10:11 PM ET (US) I never personally checked but I thought the 13 footer had a built in hook to reduce porposing. Probably Tom Clark would know. If so the bottom could be straightened to produce more speed.
Royboy	posted 04-12-2006 05:50 AM ET (US) I think you answered your own question: "with a manual jack plate with a 3-inch set back to get engine height up and [anti-ventilation] plate level with the running surface of the water as it leaves the transom." Your engine is too high. It's losing its bite on the water and dropping the bow starting the porpoising action. Having the anti-cav plate precicely level with the bottom of the boat isn't necessarily the very best place for it. The manual says plus or minus an inch as a general recommendation. Should be an easy matter for you to find the optimium running height of your engine. Roy
jimh	posted 04-12-2006 08:33 AM ET (US) Porpoising, a tendency for the bow of the boat to oscillate up and down when the hull is planing, can occur in any boat, although some configurations make this occur over a wide range of conditions. Generally adjustment of the engine trim angle will permit the boat to remain on plane and stop the bow oscillations. Weight distribution in the boat also affects porpoising. What are: --trim buttons? --gunwale trim? I am not familiar with those terms, and I would appreciate an explanation.
Tom W Clark	posted 04-12-2006 10:38 AM ET (US) The 13 footer is not known for porpoising. I have run a great number of these little boat over the years including my own 1976 Sport 13 Center Steer. The 13 footer will generally run flat and stable if set up correctly. The 13 foot hull has a hook designed into it NOT to reduce porpoising, but to help get it on plane and stay there. It may well effectively help prevent porpoising but I suspect in this case 13redwhaler is fighting some other element of his/her set up. If the cavitation plate is flush with the keel, that is good. This certainly is not the cause of the porpoising. Generally, having the motor set to low will aggravate porpoising, not having it too high. It could well be that 13redwhaler's motor could even be mounted higher given it is set back somewhat. The usual cause of porpoising is excessive rear weight bias. The set back bracket will aggravate this condition. To test if rear weight bias is contributing to the problem, try temporarily moving weight forward and running the boat again. Note of the porpoising stops or is diminished. The use of power trim on a 13 is of little value. The 13, because of its hooked bottom and flat running attitude cannot have its bow lifted much. Power trim can be used to "tuck in" the motor to help the boat get on plane but once there trimming out beyond the point where the motor is vertical will have little benefit.
Royboy	posted 04-12-2006 01:45 PM ET (US) I agree with everything Tom said except about power trim on a 13. I have it on mine and it makes a world of difference. I can raise the bow up a good foot or so and either cause, or get rid of, tendency to porpoise with trim alone. I alsways start with the engine trimmed all the way in, and once on plane trim the motor out until the hull goes quiet. It equates to a few mph increase in speed. This is a 2000 model, however. Roy
hmbbuddy	posted 04-15-2006 01:49 AM ET (US) Try using a dolfin Or what you called "wing" in your propeller area. It is a good add-on and stabilizes your run.
project13	posted 04-17-2006 09:26 AM ET (US) "The use of power trim on a 13 is of little value." I also really have to disagree here also. On my 1978 13 the power trim also makes a world of difference, not only in terms of performance but also in the ability to keep the ride angle at a point that keeps the ride smooth and dry, in both a head and following sea. I would say that when I am in the boat by myself, the ability to trim out the motor is good for at least 5 mph. When I run flat out, the boat will not porpoise when trimmed out, but at anything less than full tilt, I need to keep the motor trimmed down to prevent it. I have considered a stingray/doelfin but would really like a consensus on it before I drill holes in my brand new $4K motor. Anyone else have any thoughts on it?
Royboy	posted 04-17-2006 09:53 AM ET (US) I installed a Stingray Jr on my 25hp Bigfoot to try and help my boat plane faster. It helped marginally, but the actual fix was a lower pitch prop. I never has a porpoising problem that I couldn't fix with trim, so if I had bought the new prop fisrt, I wouldn't have a fin. Roy
Teak Oil	posted 04-17-2006 05:01 PM ET (US) Roy dont you have a 130 Sport? That is a completely different hull than Tom is referring to. 13redwhaler, your problem is likely due to a combination of too much weight aft, engine mounted too low/high, or too much trim. Modification of the hull is not necessary and a waste of time and money. You should be able to correct this without spending a dime