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Author Topic:   9-foot Tender: Porpoising

footsie posted 10-16-2006 10:50 AM ET (US)
I just tried out my newly purchased 1991 9-foot Tender and 2001 8-HP four-stroke Yamaha. I cannot stop it from porpoising at full speed. It runs fine on plane until I open maxiumum throttle. Then it starts bow bobbing. I am 210-lbs and moving my body forward fails to smooth it out. Backing off the throttle a bit eliminates the problem. Tilt pin is in the bottom hole.
Could it be caused by a rocker bottom? Tender was carried on a large craft for most of it's life.
Would a [anti-ventilation plate] appendage help?
The Judge posted 10-16-2006 02:40 PM ET (US)
Yeah. Get a whale tail thingy for $20. Problem with 9-13's is they are too short and tend to porpoise at high speeds. My 13 will buck like a bronco. I too am debating a whale tail thingy but with PTnT I wonder of needed.
contender posted 10-16-2006 03:18 PM ET (US)
is your engine shaft length correct? is the engine mounted correctly? try buying/making a long fuel hose and move the fuel tank foward.
footsie posted 10-16-2006 04:35 PM ET (US)
Shaft length is 15" as is the tender. I first jacked up the motor about 1/2-inch since that brought the bottom of the cavitation plate just above the bottom of the tender. The porpoising was bad so I removed the shims and got a minor improvement. The cavitation plate is within the limits listed in the owner's manual.
jimh posted 10-16-2006 11:59 PM ET (US)
Adjusting the engine trim is the first thing to try to suppress the oscillating bow tendency.
footsie posted 10-17-2006 11:47 AM ET (US)
jimh, I'm in the BOTTOM tilt pin hole.....I can't go any lower. If I tilt the motor up it will increase the bow oscillations. I tried shimming the top of the outboard side of the transom in an attempt to get more tuck under. It failed to do any good since the outboard portion of the motor clamp brackets tightened onto the shim but the bracket refused to tuck down to the transom but remained parrellel to the transom. How now brown cow????

The Judge posted 10-17-2006 11:52 AM ET (US)
Buy a doel fin thingy and problem should be solved.
frontier posted 10-17-2006 04:50 PM ET (US)
The motor is too heavy for the boat. It's rated for a 5 HP 2-stroke. That 8 Hp 4-stroke weighs 83 #. If you must have the speed (and your insurance company is Ok with it), find a 59# 9.8 HP Tohatsu/Nissan 2-stroke.
The Judge posted 10-18-2006 11:55 AM ET (US)
25lb is not gonna do squat, especially when your ass is right next to it.
footsie posted 10-18-2006 06:59 PM ET (US)
Problem solved.....I have a 2 stroke 8 hp Tohatsu with a home made "whale tale" for use on my square end 16' Scanoe. I replaced the Yamaha with the Tohatsu and what a difference. It's much faster and no bow hop. What fun! I am sure now that an enlarged cavitation plate is the solution! Thanks guys for the comments. Now to find a good lightweight 2 stroke 15hp.

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Author	Topic: 9-foot Tender: Porpoising
footsie	posted 10-16-2006 10:50 AM ET (US) I just tried out my newly purchased 1991 9-foot Tender and 2001 8-HP four-stroke Yamaha. I cannot stop it from porpoising at full speed. It runs fine on plane until I open maxiumum throttle. Then it starts bow bobbing. I am 210-lbs and moving my body forward fails to smooth it out. Backing off the throttle a bit eliminates the problem. Tilt pin is in the bottom hole. Could it be caused by a rocker bottom? Tender was carried on a large craft for most of it's life. Would a [anti-ventilation plate] appendage help?
The Judge	posted 10-16-2006 02:40 PM ET (US) Yeah. Get a whale tail thingy for $20. Problem with 9-13's is they are too short and tend to porpoise at high speeds. My 13 will buck like a bronco. I too am debating a whale tail thingy but with PTnT I wonder of needed.
contender	posted 10-16-2006 03:18 PM ET (US) is your engine shaft length correct? is the engine mounted correctly? try buying/making a long fuel hose and move the fuel tank foward.
footsie	posted 10-16-2006 04:35 PM ET (US) Shaft length is 15" as is the tender. I first jacked up the motor about 1/2-inch since that brought the bottom of the cavitation plate just above the bottom of the tender. The porpoising was bad so I removed the shims and got a minor improvement. The cavitation plate is within the limits listed in the owner's manual.
jimh	posted 10-16-2006 11:59 PM ET (US) Adjusting the engine trim is the first thing to try to suppress the oscillating bow tendency.
footsie	posted 10-17-2006 11:47 AM ET (US) jimh, I'm in the BOTTOM tilt pin hole.....I can't go any lower. If I tilt the motor up it will increase the bow oscillations. I tried shimming the top of the outboard side of the transom in an attempt to get more tuck under. It failed to do any good since the outboard portion of the motor clamp brackets tightened onto the shim but the bracket refused to tuck down to the transom but remained parrellel to the transom. How now brown cow????
The Judge	posted 10-17-2006 11:52 AM ET (US) Buy a doel fin thingy and problem should be solved.
frontier	posted 10-17-2006 04:50 PM ET (US) The motor is too heavy for the boat. It's rated for a 5 HP 2-stroke. That 8 Hp 4-stroke weighs 83 #. If you must have the speed (and your insurance company is Ok with it), find a 59# 9.8 HP Tohatsu/Nissan 2-stroke.
The Judge	posted 10-18-2006 11:55 AM ET (US) 25lb is not gonna do squat, especially when your ass is right next to it.
footsie	posted 10-18-2006 06:59 PM ET (US) Problem solved.....I have a 2 stroke 8 hp Tohatsu with a home made "whale tale" for use on my square end 16' Scanoe. I replaced the Yamaha with the Tohatsu and what a difference. It's much faster and no bow hop. What fun! I am sure now that an enlarged cavitation plate is the solution! Thanks guys for the comments. Now to find a good lightweight 2 stroke 15hp.