New used boat porpoising.

[ninelives1972]

I just bought a 1967 Starcraft sportsman 16' fishing boat with a 5o hp mercury and a 15 hp johnshon. Both motors are operational and run well. When using either motor the boat seems to be porpoising. The bow sits quite high and the speed is not what I expected. I am new to boating and was wondering what my options are? I would appreciated all inputs. Thanks in advance.

 

[DJ]

Re: New used boat porpoising.

Porpoising is indicative of too much weight aft (rear).<br /><br />Move as many things, a you can forward. Trim the engine all the way down.

 

[swist]

Re: New used boat porpoising.

"Porpoising is indicative of too much weight aft (rear)."<br /><br />Yep, or engine trimmed out too far. Or (and I hope you didn't get screwed) the keel is not straight....

 

[BF]

Re: New used boat porpoising.

as said 2x above, try trimming the motor down. If it is against the trim pin/bar in the picture, I'd say it's the problem (looks trimmed up quite a bit) - I'm talking about the Merc. <br /><br />I've seen people that move the trim pin up to the top when trailering to give a little more clearance to the road. Maybe that's been done.<br /><br />Also, are you running it with the top up? I wonder if those things can generate enough lift to make funny things happen (?).<br /><br />Brent

 

[John_S]

Re: New used boat porpoising.

ninelives1972,<br /><br />I have the same boat, minus two feet, with a Nissan 40C. Max hp is 40, and I think your 16' is 50 hp. I do not use a kicker. I have to adjust the trim, as metioned above, for different loads. The only time I get porposing (bow moves up & down) that can't be adjusted out, is when it is just me in the boat, WOT and near top speed. I don't have a speedo, but it is around 25-30 mph. <br /><br />When I position people in the boat, they are generally in front and middle seat. Back seat only if a room issue. This probably goes more for you, given the extra 100+ lbs of the kicker, on the transom. If you can't adjust the porpusing out, remove the kicker, and see if you can then adjust it out.

 

[Maximerc]

Re: New used boat porpoising.

Good point about the top it looks like it has quite a bit of "angle of attack" if ya had a 80 hp you might need clearance to take off. don't know too if you can move the battery forward any or/and the fuel tank. does the kicker have its own fuel tank ..?? If it does can you stow the kickers fuel tank forward till needed??

 

[ninelives1972]

Re: New used boat porpoising.

I appreciate all the great posts. It seems like it unanimous that it is probably a weight problem. There are actually two 6 gallon tanks in back and the battery. I also have some hand tools back there also. I actually think I will only use the 50 probably once in a blue moon, I might even sell it. My family and I mostly will be using the boat for fishing on a lake with limited hp of 15. The boat will probably move a lot better with it off. Let me know what you think.

 

[cobra 3.0]

Re: New used boat porpoising.

You do have the extra motor and weight at the back, but try the trim setting which is likely off. Take the pin from at the back of the motor bracket and put it into a lower hole to 'straighten' the motor(make it more vertical because the motor looks angled too much). Your bow will then come down.

 

[John_S]

Re: New used boat porpoising.

With only the 15, it will really struggle to get on plane, if it even can, especially with your family aboard. <br /><br />Outside of your lake restriction, I'd say just the opposite, keep the 50 and sell the 15. How about, purchasing a 14' light semi-vee for the 15hp on your lake, and keeping the starcraft/50hp for other lakes? If you only boat on the restricted lake, and can live with the slow speed, consider trying smart tabs to help keep bow down/transom up. The rather narrow beam of the boat, prevents using smart tabs with the kicker motor, but using single motor should fit.

 

[John_S]

Re: New used boat porpoising.

If the picture has the merc in the normal running condition, I agree with cobra. Mine is usually set level or one pin in, which will make the motor cowl tilt down, where yours apears to be angling up.

 

[Jdeagro]

Re: New used boat porpoising.

All of the posts (and most others on this subject) recognize that porpoising is caused by weight balance. No question about it! The other handling problems caused by weight balance include high bow rise at slow speeds, bow steering, and difficult getting to plane.<br /><br />Changing the motor trim will allow the prop to lift the stern more and reduce the porpoising by keeping the bow down. This will reduce fuel economy. The trim ( down) will also allow the prop to push the stern up and help the boat to plane. If the trim is not electric then this prermanent adjsutment will make the boat run bow down at cruising speeds and reduce fuel economy.<br /><br />Moving weight to the from will help reduce the porpoising, but again this permanent change in balance will reduce or change other handling characteristics (ie Fuel economy, top speed, bow steering, etc.)<br /><br />First I would check the motor trim to make sure it is perpendicular to the water surface. However the 15 HP motor is not going to do much regardless of where the trim is.<br /><br />Someone mentioned Smart Tabs. The benefit of trim tabs (any) is that you can change the hull design to compensate for the changing conditions such as boat speed, balance, water conditions, etc. And when these conditions change again the trim tabs can be re-adjusted to compensate for the new set of conditions. Smart Tabs do this automatically!<br /><br />Obviously the use of the boat will determine what is necessary, but the point is that permanent changes to the weight and balance, help fix one problem, and usually make something else worse. <br />This principle is used thousand of times every day on every airplane.