Pourposing

Drag quaestion

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]

 

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

 

Did you fix the problem

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.

 

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.

 

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

 

Pourpposing

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.

 

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

 

Interceptor plates

Tom,

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

nevd

 

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

 

(>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 %

 

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.