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Author Topic:   Set Back Brackets: Affect on Porpoising

NJCoastFlyFish posted 06-10-2007 09:05 PM ET (US)
I am considering adding a set back or jack plate bracket to my 16-foot Boston Whaler boat. One concern that arose today while boating was that with just me on board in calm water the boat is on the verge of porpoising when I trim the engine up. I can stop the porpoising by trimming the engine back down but loose some speed (insignificant tenths of a mile per hour). Would adding a set back bracket and raising the engine increase or decrease porpoising? I'm surprised this hasn't been talked about yet on this forum, at least nothing I can find.
kamie posted 06-10-2007 09:57 PM ET (US)
It really depends on the boat and how it's setup.
My 18 would porpoise slightly when I first got her. I could trim out of it but the tenancy was always there. I put a setback bracket and in slight chop she would porpoise like mad, and I really couldn't trim out at all. I left the setback bracket, swapped the engine for a heavier one and took the t-top off and the tenancy to porpoise has all disappeared.
If the boat tends to porpoise I would look at what is causing it. In my case, it was the t-top which is really not a great idea on an 18 foot boat, without trim tabs to counter the forward weight. A setback bracket may make the tenancy to porpoise worse, but if the boat doesn't tend to porpoise then adding a bracket shouldn't suddenly cause the boat to porpoise.
Sal DiMercurio posted 06-10-2007 10:19 PM ET (US)
This subject has been hashed over & over on this site.
You just have to ask Jim how to find them.
Usually raising the engine will counter porpoising but I feel the setback works just the opposite, because it puts weight farther back, which tends to add to the bow going to a frog jump.
I have 5" jack plates with 5" setbacks on both of my whalers [ 15 sport w/70 hp Johnson & "V" 20 outrage w/200 hp Evinrude ].
My 20' Outrage was wonderful without the kicker, but when I put the 15 hp kicker [approx 80 lbs ] back on the transom, it ruined the ride by not allowing me to trim very much at all because it wanted to porpoise.
I just reversed the setback [ Bob's machine shop ] from 5" setback to 2" setback to see if this helps.
I just did it today, so I haven't had the chance to test it.
The 15 Sport loves the 5" setback with the 5" lift.
I can trim her way back without her even thinking about porpoising & she handles beautiful.
I do recommend a jack plate on your rig, just make sure you keep some weight forward top counter the aft weight.
Sal
Sal DiMercurio posted 06-10-2007 10:27 PM ET (US)
I forgot to add, I have an 18 gallon fuel tank under the front seat,.....might be why she dosen't porpoise.
I can get her to porpoise if the tank is down to 6 or 7 gallons & I trim way out between 3000 & 4200 rpms, but above 4200 rpms she runs level as level can get with no porpoising.
At wot I can run the trim way back, until the prop ventilates, but no porpoising.
It changes the way she rides, turns, & comes out of the hole, plus gives a little more on the top end.
Sal
jimh posted 06-11-2007 09:08 AM ET (US)
Instructions for use of the website's search function are provided on the page that contains the search function. Learning how to use the search is simply a matter of reading the instructions.
The tendency for the bow of a boat to oscillate up and down when the boat is running on plane occurs in many boats, and it is usually related to the trim setting of the motor. The range of trim setting in which these oscillations occur varies from boat to boat. In some boats there is a wide range of trim setting which will produce the oscillation, while in other boats there seems to be a relative immunity to these oscillations. Exactly what features of a boat cause it to be prone to bow oscillation while on plane has never been scientifically determined in these discussions, however there is some anecdotal evidence that some of the factors are:
--weight distribution
--propeller
--hull form
--length of running surface when on plane
Adding a set back bracket is not known to be either a sure cure to stop bow oscillations or a sure way to start them.
Keeper posted 06-11-2007 01:03 PM ET (US)
I'm having the same problem with my 1988 15 Boston Whaler Super Sport powered by a 1988 70 HP Yamaha 2-stroke.
For me to raise the motor alone requires re-driling the transom. That is not going to happen.
My doel-fin does not stop the problem in freshwater. In salt water it's much less a problem (buoyancy)
I am close to buying a Bobs Machine Shop 5 inch set back jackplate. My concerns are that it will make the problem worse and that the transom will sit much lower than I'd like.
I have a bow mounted trolling motor adding a pretty good amount of weight forward.
KEEPER
Tohsgib posted 06-11-2007 01:36 PM ET (US)
Go for it keeper. Anytime an engine is mounted in the "blind holes" it is a MAJOR plus to add a bracket. Just drill the bracket and mount it in your old transom holes then install engine on bracket. I did it to my Newtauk...easy.

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Author	Topic: Set Back Brackets: Affect on Porpoising
NJCoastFlyFish	posted 06-10-2007 09:05 PM ET (US) I am considering adding a set back or jack plate bracket to my 16-foot Boston Whaler boat. One concern that arose today while boating was that with just me on board in calm water the boat is on the verge of porpoising when I trim the engine up. I can stop the porpoising by trimming the engine back down but loose some speed (insignificant tenths of a mile per hour). Would adding a set back bracket and raising the engine increase or decrease porpoising? I'm surprised this hasn't been talked about yet on this forum, at least nothing I can find.
kamie	posted 06-10-2007 09:57 PM ET (US) It really depends on the boat and how it's setup. My 18 would porpoise slightly when I first got her. I could trim out of it but the tenancy was always there. I put a setback bracket and in slight chop she would porpoise like mad, and I really couldn't trim out at all. I left the setback bracket, swapped the engine for a heavier one and took the t-top off and the tenancy to porpoise has all disappeared. If the boat tends to porpoise I would look at what is causing it. In my case, it was the t-top which is really not a great idea on an 18 foot boat, without trim tabs to counter the forward weight. A setback bracket may make the tenancy to porpoise worse, but if the boat doesn't tend to porpoise then adding a bracket shouldn't suddenly cause the boat to porpoise.
Sal DiMercurio	posted 06-10-2007 10:19 PM ET (US) This subject has been hashed over & over on this site. You just have to ask Jim how to find them. Usually raising the engine will counter porpoising but I feel the setback works just the opposite, because it puts weight farther back, which tends to add to the bow going to a frog jump. I have 5" jack plates with 5" setbacks on both of my whalers [ 15 sport w/70 hp Johnson & "V" 20 outrage w/200 hp Evinrude ]. My 20' Outrage was wonderful without the kicker, but when I put the 15 hp kicker [approx 80 lbs ] back on the transom, it ruined the ride by not allowing me to trim very much at all because it wanted to porpoise. I just reversed the setback [ Bob's machine shop ] from 5" setback to 2" setback to see if this helps. I just did it today, so I haven't had the chance to test it. The 15 Sport loves the 5" setback with the 5" lift. I can trim her way back without her even thinking about porpoising & she handles beautiful. I do recommend a jack plate on your rig, just make sure you keep some weight forward top counter the aft weight. Sal
Sal DiMercurio	posted 06-10-2007 10:27 PM ET (US) I forgot to add, I have an 18 gallon fuel tank under the front seat,.....might be why she dosen't porpoise. I can get her to porpoise if the tank is down to 6 or 7 gallons & I trim way out between 3000 & 4200 rpms, but above 4200 rpms she runs level as level can get with no porpoising. At wot I can run the trim way back, until the prop ventilates, but no porpoising. It changes the way she rides, turns, & comes out of the hole, plus gives a little more on the top end. Sal
jimh	posted 06-11-2007 09:08 AM ET (US) Instructions for use of the website's search function are provided on the page that contains the search function. Learning how to use the search is simply a matter of reading the instructions. The tendency for the bow of a boat to oscillate up and down when the boat is running on plane occurs in many boats, and it is usually related to the trim setting of the motor. The range of trim setting in which these oscillations occur varies from boat to boat. In some boats there is a wide range of trim setting which will produce the oscillation, while in other boats there seems to be a relative immunity to these oscillations. Exactly what features of a boat cause it to be prone to bow oscillation while on plane has never been scientifically determined in these discussions, however there is some anecdotal evidence that some of the factors are: --weight distribution --propeller --hull form --length of running surface when on plane Adding a set back bracket is not known to be either a sure cure to stop bow oscillations or a sure way to start them.
Keeper	posted 06-11-2007 01:03 PM ET (US) I'm having the same problem with my 1988 15 Boston Whaler Super Sport powered by a 1988 70 HP Yamaha 2-stroke. For me to raise the motor alone requires re-driling the transom. That is not going to happen. My doel-fin does not stop the problem in freshwater. In salt water it's much less a problem (buoyancy) I am close to buying a Bobs Machine Shop 5 inch set back jackplate. My concerns are that it will make the problem worse and that the transom will sit much lower than I'd like. I have a bow mounted trolling motor adding a pretty good amount of weight forward. KEEPER
Tohsgib	posted 06-11-2007 01:36 PM ET (US) Go for it keeper. Anytime an engine is mounted in the "blind holes" it is a MAJOR plus to add a bracket. Just drill the bracket and mount it in your old transom holes then install engine on bracket. I did it to my Newtauk...easy.