Pourposing

[CaptPPan]

My 27' center console inboard fixed surface drive boat pouposesat some speeds and RPM I can usualy correct and eliminate it with the trim tabs, but alot of trim tab deflection seams to slow the boat. I am considering installing longer tabs, or rearanging the waight somehow. Any input would be appriciated.

[mmd]

There are several possible reasons for the porpoising that your boat is experiencing. Porpoising is caused primarily by speed, excessive bottom loading (a heavy boat for a given waterplane area), longetudinal centre of gravity (LCG) location, or a combination of these factors. Some solutions are available, and of these, some are easy and some are difficult and costly.

Unfortunately, the easiest, cheapest, and most likely to be effective is the least palatable solution - slow down. Almost all dynamic-instability problems can be eliminated with a reduction is speed. Not what you want to hear, I'm sure, but sadly true.

You can use external devices such as trim tabs or transom wedges to alter the running angle of the hull, but these add drag and slow the boat down, as you have discovered.

You may want to shift some weight in the boat aft to see if this will reduce the problem. Check to see if this will work by putting a few hundred pounds in the form of buckets of water or fishing buddies at the transom and going for a spin. If you have a large fuel tank, do this when you are low on fuel so that you are shifting the LCG aft without adding weight to the boat. If this works, look for heavy things that you can permanently move aft, like batteries and fuel tanks.

If your boat is steered by a rudder(s), they may be cavitating at speed and causing the porpoising. The cure here would be to replace it (them) with rudders that have cavitation plates at the top.

If your outdrive itself is too near the surface of the water and consequently cavitating, it could be the problem. The fix would be to add an aftermarket cavitation plate above the prop.

There are further things to do, but these entail re-working the hull, and I'm betting that you don't want to go there, at least not yet.

Keep us informed about any further developments, and good luck.

[CaptPPan]

Thanks mmd
Are you saying that moveing the LCG aft will most likely help the situation? If that's so then longer tabs may not help me.Tabs wil only give me bow down trim, I thought some long ones would give me some more planing area in the rear. The drive is a fixed surface piercing so it's always cavatating or more accuratly superventalating.
I will try some water balast today.

[gonzo]

Porpoising is normal with fixed drives at some speeds. That is what the trim corrects, at a price.

[CaptPPan]

The water balast may have answered some questions. I used two 10 gal pails filled with water. I started with them in the back and the pourposing was worst then ever. I moved them to the bow and It was like a different boat alltogether, just a beutiful smooth ride. In a folllowing sea with about 25knot wind and 2-3ft swell the bow did plow alittle.
I think the answer will be longer thiner tabs, I have short fat ones now. That way I'll get the trim I need and with alittle less drag. The short fat tabs are like puting the flaps down in a plane while flying.

[Willallison]

That makes more sense to me - I was always under the impression that porpoising was the result of the LCG being too far aft......

Changing the tabs may well fix the problem, just as adding weight fwd will do. But how about onboard weight.....why not look at moving batteries, stores and the like further fwd. Fuel and water tanks can be shifted too, though its best to have them located as close to the LCB as possible, so that trim isn't affected as they are filled / emptied

[Guest]

I agree with you Will. I think MMD had jumpy fingers on the keyboard since I'm sure he knows this stuff.

Tom L

[gonzo]

What was the behaviour of the boat at different speeds? Did it solve the porpoising at the top end and make it worse at cruising speed?

[mmd]

"MMD had jumpy fingers on the keyboard " - <lol> more likely jumpy brain functions from trying to post a serious answer at 2:00 a.m. Sorry for the confusion, glad a solution presented itself.

The balance of a boat on plane is such a delicate balance between LCG, centre of dynamic lift, and the hull form. All three must work in concert to provide a stable planing condition. Such subtle changes as a small change of a running strake's size or angle can alter the whole character of a hull's handling behaviour. What I have always found fascinating is that what is a reasonable solution for one hullform is an anethema in a different hull. As a book-end to the solution found for CaptPPan's boat, consider the following anecdote:

A local boater had a porpoising problem with his 7-metre speedboat that was aggravated by light chop. I looked at the boat and thought that there was a preponderance of weight forward, which might be causing the hull to cycle between accelerating up the dynamic lift C of E until CofE passed under the LCG and the boat "fell off the wave". It would then immerse more hull causing additional drag and slowing the boat down, but the shape of the hull quickly added more lift and the bow went up, the boat accelerated, and the cycle began anew. In light chop, when the bow dropped into a wavelet, the additional lift and buoyancy seemingly "bounced" the hull back up with increased vigour. In the proper conditions, the magnitude of the porpoising increased, even though the throttle setting was constant, until it became quite alarming. We used the highly technical and sophisticated water-bucket trial method, and determined that by placing about 50 kilos about a meter ahead of the transom the resulting shifting of the LCG aft eliminated the problem. He relocated his batteries and solved the porpoising problem.

CaptPPan, if you can still trust me (it is 2:00 p.m. now, so I am more awake <others may refute this>), I'd look at relocating some weight rather than swapping trim tabs. More tab - regardless of shape, will just add more wetted surface and appendage drag and though it may solve the porpoising problem, it will come at the cost of additional expense and lower top speed. Shifting some weight already in the boat probably won't cost near as much as a set of trim tabs, and the top speed should remain the same or maybe even get a bit better.

[Guest]

Sure enough MMD,

In engineering, this porposing behavior would be classified as simple harmonic motion. The solution is to damp out the oscillations and the best solution is to do this without causing more drag. As you said, this sometimes calls for moving weight forward and sometimes aft. It's more common to have weight too far aft though. The number I've heard and used as a guide is to have the CG at least 1 1/4 times the max waterline beam forward of the transom. (I think I got that right)

Of course, the hull shape forward and aft will have an effect too.

Tom Lathrop

[CaptPPan]

The pourposing was mainly at cruising speed, when i gave it more throtle the bot flattened right out and the bow stayed down, at speeds much not to much slower then cruise the prop(about 2'behind the transom) would fully submerge and it does'nt seam to like that.

I can move the batterys, that would be easy and I think thats a good idea. I'm also adding some more encloseure that will add some weight.

If the weather is better tomorrow I'll try and take some pictures of the boat at speed.

[CaptPPan]

Sea Venture at speed

[gonzo]

The picture doesn't load. Could you post it again?

[Jeff]

CaptPPan - for some reason the photo you posted got truncated and the bottom didn't make it to the server - I resaved the top half in your post above at least - hope that's ok.

[CaptPPan]

Jeff, I don't know what any of that means, but thanks.

[DWS]

Hope you can help. Buddy and I have an ongoing discussion about which propulsion system has more hydrodynamic drag, an inboard or an I/O. Question is - excluding propellar effeciency and trimability of the I/O, which system presents more drag to forward motion? Put another way, if the propellars were removed, the I/O in it's normal trim position, and both boats towed at the same speed, which would be harder to tow?

Either a direct answer or directions to references is appreciated. Thanks.



UOTE=mmd]"MMD had jumpy fingers on the keyboard " - <lol> more likely jumpy brain functions from trying to post a serious answer at 2:00 a.m. Sorry for the confusion, glad a solution presented itself.

The balance of a boat on plane is such a delicate balance between LCG, centre of dynamic lift, and the hull form. All three must work in concert to provide a stable planing condition. Such subtle changes as a small change of a running strake's size or angle can alter the whole character of a hull's handling behaviour. What I have always found fascinating is that what is a reasonable solution for one hullform is an anethema in a different hull. As a book-end to the solution found for CaptPPan's boat, consider the following anecdote:

A local boater had a porpoising problem with his 7-metre speedboat that was aggravated by light chop. I looked at the boat and thought that there was a preponderance of weight forward, which might be causing the hull to cycle between accelerating up the dynamic lift C of E until CofE passed under the LCG and the boat "fell off the wave". It would then immerse more hull causing additional drag and slowing the boat down, but the shape of the hull quickly added more lift and the bow went up, the boat accelerated, and the cycle began anew. In light chop, when the bow dropped into a wavelet, the additional lift and buoyancy seemingly "bounced" the hull back up with increased vigour. In the proper conditions, the magnitude of the porpoising increased, even though the throttle setting was constant, until it became quite alarming. We used the highly technical and sophisticated water-bucket trial method, and determined that by placing about 50 kilos about a meter ahead of the transom the resulting shifting of the LCG aft eliminated the problem. He relocated his batteries and solved the porpoising problem.

CaptPPan, if you can still trust me (it is 2:00 p.m. now, so I am more awake <others may refute this>), I'd look at relocating some weight rather than swapping trim tabs. More tab - regardless of shape, will just add more wetted surface and appendage drag and though it may solve the porpoising problem, it will come at the cost of additional expense and lower top speed. Shifting some weight already in the boat probably won't cost near as much as a set of trim tabs, and the top speed should remain the same or maybe even get a bit better.[/QUOTE]

[CaptPPan]

After the water test proved I needed more weight in the bow I looked at what I could move. Since my batery's are at the transom and thats 2 feet behind the planing surface ( like mounting the batterys on the swim platform of any other boat) I figured that would be a good place to start.
I do plan on building some more weather protection in front of the console that will add some weight in the bow, but for now an 80 lb bag of Porland Cement up buy the anchor chain locker is keeping the boat trim and stable.

Thanks for all the help

[nicznz]

Did you manage to fix the problem of porposing if not I may have a very quick and easy solution that we sometimes use on real shockers that works every time

[CaptPPan]

Did you manage to fix the problem of porposing if not I may have a very quick and easy solution that we sometimes use on real shockers that works every time
I'm in the proess of haveing a new prop and driveshaft made. The prop guy tells me that the one I was running had way to much "rake" and that was lifting the bow. He seams to have some expereance with surface drives so I'm giveing it a shot. I needed a new prop anyway and he's getting me one with less rake.

What did you have in mind.

[nicznz]

We had one a 23ft cabin cruiser style of boat that a guy brought in for us to look at. We took it for a burn and once we got [past 22mph it just started leaping out of the water and got worse until we had to back off or drive it fully trimed in (not a pleasurable experience in either mode)
Luckily the boat had a trailer so we pulled it out and with a little bit of filler we filled from the strake in towards the centre of the boat about 4 inches long and going forward 50mm fading in to nothing. this took about 30mins to do and fixed the problem straight away .it is a bit of a cheats way of solving the problem but it works a treat . if you would like a photo to explain it better just sing out.

[adamwoj18]

i am looking at buying a new hydrasport and i hear that they porpose alot. its a 28ft wa....what exactley is porposing..please help

[tom kane]

A fast boat that starts porpoising usually has insuficient wetted surface to support the boat .Interceptor plates are a relatively new idea.That is metal plates which protrude at maxamim 1 inch below the bottom of the hull at the transom.There are different methods of fitting and raising and lowering.They may be static.They may be much more use than trim tabs,can work faster and some claim have less drag than trim tabs.They create a different pressure distribution to trim tabs.Pressure is on the hull rather than on the trim tabs.

[tom kane]

Interceptor plates www.nautica.it/superyacht/494/tecnica

[nevd]

Tom,

I get a "403 forbidden" message when I try to access this site.

nevd

[tom kane]

Try typing in www.nautica.it/Superyacht/494..in your web address bar or try a Google search in Images..interceptor plates boat.

[RANCHI OTTO]

(>DWS)

According my own experience the shaft on line (inboard) has less drag . The difference is greater at low speeds (20 knots) and lower at high speeds (40 knots) btw. 12 and 4 %

[yipster]

Tom, coincidentally i was reading bout interceptor plates also. thought it was a rather crude way of contolling but saved the pic. not the site

[tom kane]

Tom, coincidentally i was reading bout interceptor plates also. thought it was a rather crude way of contolling but saved the pic. not the site


Like all things,you can have basic or very sophisticated models.It pays to realy check out things well before decisions about their worth is made.www.nautica.it has a lot of info.