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Discussion Starter · #1 ·
On Sat, when I went out to wet the lines, the boat would porpoise when ever I reached anything above 3300 rpm. Last time I had it out, it did not do this. The only difference was that I filled up the boat. It was about a 1/4 empty. I played with the trim as much as possible, but no luck. I tried to move up all the weight I could into the bow, but it changed very little. I have researched a few different options, including going to the boat tomorow and ensuring that the Cav plate is equal to the bottom of the boat. Also adding a "Whale tail, Sting Ray, Etc." or even adding trim tabs like these
http://www.boatownersworld.ca/smart_tabs/sx.htm.

When I took it out for the sea trial, it never did this, but also there was two of us and he weights about 280 (power lifter shape) and I am 220 (.......Shape). Could it just be a weight problem, too much in the stren and because it is aluminum, too light in the bow?

Any help would be appriciated.

Boat - 18" Hewescraft welded aluminum with a 34 gallon fuel tank, 115 yammy four stroke and 8 HT yammy 4 stroke. Also two batteries and downrigers, all in the stren.

Sorry for the long post.
 

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You mught need to re-mount your engine. Not very hard to do. I'd guess that it probably needs to be lifted a little bit. Another more costly option is to add a jack plate to the boat. This way you can adjust your height easily whenever you want, depending on the load you have.

Good luck, Wes
 

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Before I'd go and remount the engine Sitka, I'd try to determine if there was something simple that might be overlooked. I presume your main engine has an electric tilt. I know this sounds dumb but can happen to anyone...are you 100% certain that the engine was in the fully down position when you last operated it?

I've had two boats (15 foot Harbercraft and 20 foot Northriver) that had to go back to the factory to cure a porpoising problem. They made what looked to be very very minor changes to the stern of the boat and voila!.. no more porpoising. During the 'excitement' of your initial test drive, maybe you overlooked the porpoising or downplayed the extent of it? I don't mean to insult your intelligence Sitka because this can happen to anyone. Just looking for answers.

If placing a good portion of the weight forward did not cure the porpoising, I would think its one of two things then - either incorrect trim of the o/b or a defect in the hull. The boat you have is more than big enough to handle you two big guys so its not your weight thats the problem. Just my two cents.
 

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Like Rivahman said, check to make sure the trim is going all the way down. Trim tabs do help, but you want to eliminate all the other little things first.
 

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With the size and power you have weight wouldn`t be a problem. It definately is a problem with the trim. Someone must have reset the trim position or you did not have the trim all the way down. If someone rest the trim position it can also be too far down. Incorrectly setting the trim is the most common problem.
 

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Sounds like a lot of weight at the back of the boat for that size.....did you by chance try to run wide open for a stint? The faster you go the less boat needs to be in the water to support the weight and the porpoise problem should go away. The engine height is not the problem the engine angle is. Check to make sure that the engine is in the full down position and on that particular motor there is a trim pin that sets the lowest trim position which can be moved so that the engine can trim under even more. I would check the following before adding more toys (ie trim tabs or hydrofoils)
1-- Make sure there is no warp in the hull. use a long straight edge for this!
2-- Make sure the motor is in the full down position and that the trim pin is the lowest hole possible
3-- Take the down riggers out and all the removable gear you can and go for a ride by yourself to see if the problem persists. Make sure you run at wide open for a length of time to allow the bounce to fade out.
----if it still porpoises----
4-- the hull on the majority of aluminum boats extends past the transom plate and can be bent down ( very very little) with a good size of cresent wrench...small adjustment then test then readjust again if required.
5-- Put your average load back in the boat and retest. And readjust if needed.

If the boat still porpoises after this then trim tabs are the next step ( I suggest the auto tabs...cheaper, easy to use, no constant adjusting)

or you can stop by my shop any time and I can take a look for you and make some suggestions

cheers
steve
 

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Just saw a pic of your boat on another thread Sitka..nice machine! There is no way, with a boat that size, that 115 hp and 8 hp outboards would equate to a stern heavy situation, that would cause your porpoising problem. No way in the bloody world. Look around any boat yard and show me ANY new 17 or 18 footer that isn't equipped with AT LEAST 100 hp these days, if not upwards of 150 or 175. Forget the weight of the 8..its neither here nor there. I try to stay pretty openminded here Sitka but there are times that I just can't hold back :wallbash: so sorry for the outburst. It is plain bogus advice in my opinion and doesn't help you at all to suggest that the outboard (s) you have are too heavy for that boat. What are you supposed to do now? Sell the boat or sell the big one and buy a 75 or 90 hp? Go ahead and do that and I virtually guarantee the porpoising will still be there.

I don't know if you have fixed the problem or not but if its not the obvious of incorrect trim angle on the 115, I would bet with all the pennies in my savings, it is a small hull defect that can be easily fixed. I would take it back to the dealer, if you haven't already, and they can ask for factory assistance if need be to correct it. In my humble opinion, I don't believe trim tabs are the way to go either. Trim tabs are normally used on heavy boats to (a) get them up to plane faster and (b) once on plane, to adjust the angle of the bow to optimize bow angle for either planing trim or sea conditions. If you've ever boated in open sea conditions (ie west coast Vancouver Island or the Charlottes for instance), you will appreciate how trim tabs can be adjusted to help soften the ride.

I once had a 26' Bayliner with a 396 that was an absolute pig to get on plane. I like pigs generally so don't get me wrong there but this one was getting me down. Depending on the sea conditions, it would sometimes seem to take forever. That old sow would raise the nose with full throttle and literally minutes would go by before the nose evened out (well seemed like minutes!). Installed the trim tabs and that baby would lift..smell the salt air and drop down ever soooo nicely :thumbup: Oh I love pigs! Trim tabs are expensive, ugly on most boats and I think in your situation are totally unnecessary.

Like I said in an earlier post (and maybe I was just plain unlucky) but I have had 2 boats (among too many), a 15' Harbercraft prop and the 20' North River jet I now have that had porpoising problems that were cured by small adjustments made to the stern of the boat. The main point I am trying to make is and this is from my experience only (so take that for what its worth) ...YOU DO NOT HAVE A WEIGHT DISTRIBUTION PROBLEM! I would bet my whole life's experience in running boats (from 12 footers to 85 footers) on that. I don't know at what speeds you tested the boat and at what point it started to porpoise but, in the experience I've had, porpoising motion (once the boat was on plane) increased with either sustained running at one speed or with an INCREASE in speed. In fact, what would start out as very mild porpoising would increase in intensity when the speed remained the same over time or if speed was increased to the point that you'd have to throttle back and start again. As a side note, I had a truck once that although it didn't 'porpoise' as such, built up a harmonic motion (light bouncing) in the front springs on speeds between about 25 mph and 29 mph, that would increase in intensity if you drove too much at that speed. Increasing or decreasing speed below or above 25-29 would make it go away but its clear the harmonic motions exhibited by porpoising boats are not limited to boats.

I'm sure alot of us have seen those boats just pounding..err porpoising over the water. I just shake my head at the harmonic motions of those boats and feel for those people because they don't know how easy it can be to fix the problem. Don't be discouraged Sitka..get it checked out by the dealer or the factory if need be and it will be fixed. That I can guarantee and you will be a much happier boater :beerchug:
 

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I agree rivaman I had that prob once and yes adjusting the weight helped but did not fix the prob till I added a set of dol-fins never had a problem again and that old fiberglass boat handled way better, I don't have the boat anymore but the fins solved my problem.
 

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Sitka,
you have answer in your first post:

"On Sat, when I went out to wet the lines, the boat would porpoise when ever I reached anything above 3300 rpm. Last time I had it out, it did not do this. The only difference was that I filled up the boat. It was about a 1/4 empty."
With that much weight you cannot reach ideal balance between POM (point of mass) and POB (point of balance) :2cents:

:cheers:

B.
 

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According to Hewescraft specs, Sitka's boat is rated for 150 hp max, which is more than the 115 he has on it now. The fuel tank is built in and fuel tanks ARE designed to be filled up. He does not have a weight problem!
 

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Rivahman said:
According to Hewescraft specs, Sitka's boat is rated for 150 hp max, which is more than the 115 he has on it now. The fuel tank is built in and fuel tanks ARE designed to be filled up. He does not have a weight problem!
plus this: "When I took it out for the sea trial, it never did this, but also there was two of us and he weights about 280 (power lifter shape) and I am 220 (.......Shape)."

B.
 

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Discussion Starter · #13 ·
Thanks everyone.

I am going to try and get out again this weekend, if the weather permits, and try again. I will let all know how it went.

I have talked to a few others and it is funny how many different opinoons there is out there.

Cheers

SS
 

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17-18 foot boats are typically designed to carry 4 people. 4 people @ 150 lbs each is still less than those 2 big guys combined. The seats in that boat are about amidships. Unless one of them was standing at the stern while underway at high speed (highly unlikely), much more weight is on the forward part of the boat than typically by a couple of 150 lb runts. 500 pounds of people weight amidships more than compensates for any excessive weight at the stern (which there isn't).
 

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There are many things that cause porposing and weight DISTRIBUTION is one as well as speed, trim angle and hull shape. Before anything can be figured a few things have to be determined first. The trim angle and hull shape have to be considered first and foremost. Then the difference between a load that porposes and one that doesn't has to be figured out. From there it can be figured out what is causing the porpoise.
I have a thing for the old school flat bottoms, bubble decks and 16'- 17' speedsters. You can put 200HP on the back and with those hull shapes at slow speeds they will porpoise..always...but once you get moving then the boat weight is supported and the bounce goes away. This is the same with the 1725 NE (harbercraft aluminum with a 135 verado) I ran last summer. If the bounce gets worse then you are not moving fast enough (for those hull shapes) and more HP is needed (no replacement for displacement!) :thumbup: If the HP is set then the hull must be set for running at a set speed. In your case rivahman, the extened hull would be tweeked down to provide more lift in the stern thus keeping the bow down. This can only be done so far and is a standard and quick fix. One thing about some of these boats that are designed for both prop and jet the hull is set in favour for one or the other (jets work best with a slight hook and props with a slight rocker, VERY Small!) and is why these units need some tweek when the other propulsion type is installed.
On the other hand, when a boat works perfectly fine with a small load and porpoises with a larger load than, assuming all other factors are the same, the load PLACEMENT is the problem. The motor weight is not an issue because the bigger the motor the faster you will go and the larger the prop thus a greater ability to carry weight and provide stern lift. Porpoising is all about stern lift provided by hull and water speed and influenced by wieght distribution relative to the center of mass or center of gravity of a vessel, the hull angle relative to the surface of the water and the amount of weight that can be "carried" by a prop. (the reason why stainless props work better and give better top end is because they hold their shape under load and can carry the bow of the boat higher thus leaving less boat in the water and less friction so allowing more speed with the same Hp.)
The best thing for you to do sitka is put the average load (what you will carry in the boat 90% of the time) and test and adjust the boat accordingly. Or put this load in the boat and have the dealer make the necessary adjustments. Trim tabs are the absolute last resort and they are placed on hulls from 10' (sport tabs) on up, but standard on anything over 18 degree deadrise hulls (adjusts for "list") or over 20' long deep vee hulls operating in large seas. The Hyrofoils or dolphins that are installed work as well and I would put those on before trim tabs anyday, (shaped like an airplane wing to generate more stern lift) however I doubt that those are needed. Put a standard load in it make sure it trims under and adjust either trim or the hull extension as needed.
 

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Sitkaspruce,

before you take a look on this video imagine this:


seesaw is your boat,
your legs are your boat engine,
your friends are weight added to your boat (fuel, batteries, downriggers, beer or ...etc)
Mad bull act as a sea waves,

http://www.dailymotion.com/video/x3efba_klackalica_fun


See what is happening???????????? A lots of porpoising if weight is not distributed properly!!!!!!!!!!! :happy: :happy: :beerchug: :drunk:

Cheers,

B.
 
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