What causes Porpoising

[htillman]

Ran my new to me 20 today for the first time. It is powered by a 150 ETEC. The engine only turns up 5000 rpm and seems to porpoise too much. I had to trim the motor down and run with the trim tabs down to get the boat to run smooth.
What causes this? Could the problem with the porpoising and the motor not turning up be due to the height of the motor?

[Bushwacker]

Porpoising will tend to occur if boat is stern heavy or motor is trimmed out too far. My boat, which is definitely stern heavy with the bracket, will tend to porpoise if I trim motor out to more than about 50% on I-Command gauge, and that's with a good 4B prop with lots of stern lift. How much do you have to trim down to stop it from porpoising? More weight up front would probably help.

What prop are you running? When I looked at that boat it had a Rebel on it but I couldn't tell what size it was. That's a good stern lifting prop, but if that's what you're running, you need one with less pitch, as 5000 rpm @ WOT is way too low. Optimum rpm on that motor is 5500-5600 so you probably need to drop down a couple inches in pitch.

Is engine height correct? (Should be able to see top of AV plate when you're up on plane and trimmed out as far as you can go w/o porpoising.)

[dave s]

Check the motor height first; anti cav plate should be above water when trimmed for running.
My buddy had a 25' Fountain with single 225 Optimax, that would porpoise like crazy in any seas above 6", yes 6".

We tried everything, raising/lowering the motor, four blade prop, moving the batteries forward, and adding a heavy anchor, chain, and rode forward.
Boat only rode well when the front fishbox was loaded with ice and fish, like a large swordfish.

The thing that finally worked the rest of the time was adding 200 lbs. of sandbags to the bow locker.

[McGillicuddy]

You have 2 primary problems:
1. The motor is much heavier than the hull was designed for. Great motor, so adapt or go smaller, losing top speed but gaining handling. This could be unreasonable so go with what you have.

2. You have the wrong propeller. This is workable.

You may have an 3rd problem, that being the height of the motor.

Your boat was designed for a 300 lb motor. Yours weighs around 435.

If your highest RPM is 5000, you are way over-propped, meaning your props pitch is too high by about 3 "inches" or more and/or the shape of your prop is likely wrong also.

You need to tell the gang everything about your propeller. Make, model, diameter, and pitch. As well as what it's made of. Also what your maximum RPM is at wide open throttle when you have a pretty full tank of gas and what you would consider a heavy load. Are you're batteries astern or under the console? You'll probably need to place them as far fore as possible.

Also have a look at how high the anti-cavitation plate is relative to the bottom of the keel (use a straight edge with the plate trimmed parallel to the keel. That plate needs to be at least an inch and a half above the keel. Once you get the rpm and right type of prop, you can tweak the height so the plate is visible when running at plane. Some guys with 4 strokes and heavier 2 stokes have their motors mounted higher.

Armed with that information we can help you out.

Without that knowledge its a crap-shoot.

I will tell you right now that you will likely want a 4 blade prop or a three blade designed to lift the stern.
A lot of this info is redundant so my apologies.
Good luck and keep us posted.

[JohnC]

I think that the best overall performance comes from the prop and the running surface being parallel with one another and both being as close to parallel with the water surface as possible. Tabs are handy but they reduce speed and efficiency. I would try to move as much weight forward as you can first, then use tabs to get the rest of the way there.
And now for the unsolicited advice from the peanut gallery!
At the risk of stating the obvious I will point out that porpoising happens when the running surface of the boat becomes shorter at speed and it is directly related to weight distribution. It is in essence an attitude problem! With a stern-heavy rig the running surface will quickly shorten to the rear as the boat accelerates, leaving the bow suspended out of the water and the aft portion of the hull in the water. The dynamics that lift the bow out of the water through acceleration are reduced as the boat speeds up and less hull remain in the water. The forces that lift the bow are eventually less than the force required to suspend the weight of the bow in midair so the bow falls. After the fall more hull is in the water creating more lifting force on the bow and up she goes again! Most of the time the weight distribution problem is caused by too much motor weight. Even in new boats that are “designed” to carry larger motors there is a marked “squat” when the boat is floating at rest. It is only a guess on my part but I believe that over the past decade or so, manufacturers have beefed up their transoms to accommodate heavier motors (more HP + 4-strokes) without redesigning the overall weight distribution to optimally support and balance the extra motor weight. Just strap on a big motor and add tabs as standard equipment and voila! Not a bad fix really. Boats with big motors and trim tabs are easy to sell (and that is after all the whole point to making boats).
Basically anything that can cause the boat to run more level and maintain a longer running surface will fix porpoising problems but it is worthwhile to consider the side-effects of the fix. You could install a smaller lighter motor to create a well-balanced rig (I know, “get thee behind me, Satan”!). I think this is actually the best solution when considering new power but I am probably in the minority with that. It certainly is not a viable option to fix a little bow bounce on a working boat. You could add trim tabs. These work by trading drag for stern lift. The cost is some efficiency (in my case, with full tabs, I lose about 400 RPM and 5-6 MPH at WOT while still burning the same amount of gas). I use my tabs to even the load left to right or occasionally to lower the bow when running into chop. My rig (not a Seacraft) is pretty well balanced so I don’t need tabs most of the time. Someone mentioned adding sand-bags for weight in the bow. I think that too is a tradeoff but probably a little better one than buying and installing tabs; it creates more drag from added weight but still keeps a flat running surface. That seems like less drag than trim tabs would create. Another fix is the prop but I honestly don’t know how that works. I don’t understand how a round prop facing forward along the planning surface can create upward lift at the stern but I don’t know much about props. Someone more knowledgeable that I can explain that.
I only mention all of this because this forum is built on members who are enthusiastic about Seacraft hulls. This innovative design still has a lot of advantages when compared to similar modern boats. The Seacraft advantage is below the water line and it runs bow to stern. If a boat is stern heavy it drags a V-shaped groove on the surface of the water that is deeper and wider than the same boat running the same speed with a well balance load. You’re basically driving a bigger wedge into the water with the bow up than you are with a more level riding boat. If you are going to pile lots weight and HP on the transom then the Seacraft “flying wedge” really isn’t any better than any other “flying wedge” out there, since the stuff that makes it a great hull is suspended in mid-air.
My 2₵!

[Bushwacker]

Great analysis JohnC!

As far as props are concerned, I was told by the folks at PowerTech that a 4B prop will always develop more stern lift than a 3B prop simply because of the extra blade. However the blade design and shape can also make a lot of difference. I've read that props with more rake to the blades are supposed to create bow lift, while props with less rake generate stern lift, but I'm sure there is more to it than that. I have a 14 1/8 x 21P 4B Michigan Apollo that is supposed to be a bow lifting prop that I can trim out to about 75% before it starts to porpoise, but my stern lifting 15 x 15 4B PowerTech prop will start to porpoise over about 50% trim, so there are probably a number of variables involved. Ed, it might be worth a call to Ken at Prop Gods to see what he would recommend for Pitch/diameter and style of prop to minimize the porpoising problem.

I have checked the prop shaft angle vs engine trim and it appears to be parallel to the keel at 50% trim, so I don't think there is an efficiency penalty for running at that trim setting.

[htillman]

Thanks all for the feedback. I'm going to closely inspect the prop currently on the motor and do some measuring on engine height. I really like the ETEC and dont want to change the engine. Moving the console forward would probably be a good idea but would require a lot of work, I'm also going to look into this to move weight forward. The batteries are already mounted under the console as far forward as possible. I get some pics or what I have this weekend.

[JohnC]

Another thought! One of the things I would like to have (not yet - still shopping for a hull) is a fresh water wash-down. If you have access to the space under the casting platform that might be a good place for some water tanks. One big one may not fit but two of the 10-15 gallon sizes piped together would probably fit under the platform if that is possible for you. 20 Gallons of water = 166.8 pounds. That could be a creative and useful way to ballast the bow.

[Bushwacker]

Or install a sea water ballast tank in the bow like Moesly used in his raceboats that you could fill or empty as sea conditions warrant. http://www.moeslyseacraft.com/upload...st-3503358.pdf

Could use a rubber bladder type which would eliminate need for a vent!

[eggsuckindog]

I have a 415lb 200 on my 20 and 20 lbs wouldn't make any difference - the motor is prolly just too low. Now the prop does have a bering on this as well the Stilletto I have on mine has been continually been raised until its now close to 2 1/2" above hull bottom and its runs great, 3"+ was too much. My other prop wouldn't trim for squat 2 holes lower and made the boat wet as hell, I firmly believe the Stiletto is the best prop for a 20 SeaCraft - a 17 would work for you, my 200 had trouble swinging a 19. JMHO but is pretty good - just say'in