porpoising" need help"

[rodster]

Have a 18ft. fiberglass with v-front and flat bottom; 115hp with hydro foil fins at the cavatation plate. Cannot go OWT with out porpoising or cannot avoid porpoising at top end speed. the bow goes up-and-down even on flat water. note the mechanics lower the motor at its lowest seeting assuming it would help. I haven't tried going the other way by elevating the motor? <br />The maximum trim angle at OWT could only described by the top of the merc cover is approx zero degrees (flat). Any little ajustment with the trim i start porpoising. At slower speeds the trim angle can be more generous. i've noticed on most boats the motor cover angle is pointing down towards the front of the boat. Do i have real problem or am i being greedy with the trim and acceleraion.<br />note: my maximum speed is 48mph with porpoising and without porpoising its 43mph..thats a big difference. HOw can i achieve max speed without porpoising?<br />Any suggestions guys.

 

[ob]

Re: porpoising" need help"

You'll have to get it trimmed down more for your boat application or 43 is going to have to be good enough. If boat planes easily consider removing plate fins

 

[Forktail]

Re: porpoising" need help"

It sounds like you have too much trim. Either the boat's transom isn't angled enough, the hydro fin is reacting too much, or your weight distribution in the boat is all wrong. <br /><br />I'd try removing the hydro foil fin and moving some weight up front.<br /><br />Good luck.

 

[rodster]

Re: porpoising" need help"

the fins off or on i ca't tell the difference but i will try again. The boat is open deck bass boat so can't distribute weight to the front as easy a some due to lack of storage space. is there a another solution in the lines of higher or lower motor setting or special prop? i will try adding weight to the front but the boat is already a heavy evenly load . thanks guys

 

[Skinnywater]

Re: porpoising" need help"

I've heard that a jackplate will remedy this problem.

 

[MGuckin]

Re: porpoising" need help"

Need more forward weight or more tuck.<br />A jackplate or wedges should help.

 

[longdude17]

Re: porpoising" need help"

well all of these remedies propably will work. But I have a suggestion, how old is this boat? since you said that it was fiberglass thats why im asking. I had a friend that had an old fiberglass boat and it was doing the same thing . the cause was that Because the boat sat on the trailer so much with only two support boards under it the hull over time kind of bent a little. the weight of the motor caused this (also he didnt take care of the boat). this is very uncommon but possible. I couldnt tell by looking at hte hull by I was assured my more than four boat specialist that this is what happened. they said that when the boat is new everything is strong, and perfect as the boat ages things get weaker, and becuase of the trailer and pulling the boat on the road with out a transom saver, the motor caused the boat to bend a quater on an inch. this threw off the design of the boat and messed up how it rides. At least this is what im told.

 

[RICKRICK1]

Re: porpoising" need help"

If you have a forward livewell try pumping some water in to add weight.

 

[rodster]

Re: porpoising" need help"

weight in the front won't solve the problem 100%, the boat is to heavy to make that much more of a difference. I had my friend at the front of the boat and it didn't solve anything.<br />The jackplate might be the solution but you haven't told if the jackplate will help by the motor lowered or moved higher.<br />thanks guys

 

[ob]

Re: porpoising" need help"

Rodster cavitation plate on motor should be at or near bottom of boat surface at transom.Trim angle on motor if adapted to transom properly should be able to cause boat to plow when trimmed fully down.Perhaps without investing in jack plate you can install transom brackets that will position motor as such that will allow you to trim motor down further and solve problem.

 

[dihwp734]

Re: porpoising" need help"

I have an 18' Hewes with an F100 and I had the same problem with the porpoising. I use the trim tabs to even out the ride and I switched to a diierent style SS 4 blade prop that has more stern lift. Switching props actually improved the overall ride and performance of the boat.

 

[DJ]

Re: porpoising" need help"

rodster,<br /><br />You may have to be satisfied with 43. Some hulls are just prone to this.<br /><br />My last boat was a relatively flat bottomed "bay boat" and did the same thing.<br /><br />As another mentioned, there may be a "hook" in the hull. These are very difficult to repair.<br /><br />Also, more negative trim may help. I could not rum my bay boat over 80% throttle, unless the engine was trimmed down to within 20% of the bottom (negative) trim setting. At that point it became fruitless to advance the throttle any more, I just burned more fuel and made more noise-no speed increase.

 

[Combee]

Re: porpoising" need help"

Rodster,<br /><br />Trim tabs will solve your problem. I don’t know if a jackplate will fix the problem or not. <br /><br />Email the guys at Bennett Marine Trimtabs@earthlink.net they have great customer service they’ll help you out.<br /><br />Dan

 

[suzukidave]

Re: porpoising" need help"

The flat hull might be the cause but I think trim tabs should make the problem less and you should at least save gas and go faster at same rpm from not having to trim the leg in tight.<br /><br />if bennett adjustable trim tabs are too pricey try nauticus self-adjusting trim tabs which cost about a third as much. They are very good quality and work great on my 16' boat -- i do not miss the lack of adjustment even in rough water. They make'em for up to 20 footers.

 

[Skinnywater]

Re: porpoising" need help"

rodster,<br />I was informed that the jackplate helps porposing because of the physical dynamics of motor setback, not the raising/lowering capability. <br /><br />There are added benefits as well, handling and holeshot. Also because of motor setback your able to raise the motor as much as 2" higher resulting in less drag.<br />A manuel jackplate(set it and forget it) would fit the bill and be most economical ($200).<br /><br />However, I have to <un>qualify my response by stateing I don't know this by personal experience, yet.<br />It was only in surfing the net and learning about jackplates, why, what, how and when, that I am repeating what I've been told.<br />Asking our opinions is valuable and supplies you with several options and ideas. <br /><br />The suggestion to ask the experts is most valid.