how much hp to plane?

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Apr 11, 2008
Vessel Name
Vessel Make
DeFever 48
Our boat is a 36 MT sedan with a standard semi-displacement lobster boat style hull. My question is how much additional hp over our 120 would it take to get the boat up on plane? 36x12x3-8. 23000 displacement.
On they have a prop and HP calculator. Ask that same question there - many "lobster" designs are members. For a comparo go to NT website and you'll see the 37 is about 375 HP for a 15 - 17 knot top end.

Of course the fact that your vessel *is SD does not mean it can efficiently plane. The naval architect should be able to tell you, if there was one involved in the design
I've been thinking about this and there is quite a bit than I'd like to add. First reaction is " if you want to plane get a boat designed for it". There are unlikely variables that may open the door to planing speeds for Daddyo's boat. Largely what makes a trawler a trawler is weight. There is really no such thing as a trawler yacht. They are all cruisers** ... light cruisers and heavy cruisers. Our trawlers are among the latter. The two most important elements to planing performance are light weight and a rela tively straight and shallow angle of quarter beam buttock line (QBBL). The QBBL line is a line from amidships to the stern half way from the keel to the chine (or turn of the bilge). If the QBBL is quite straight and only 2 or 3 degrees, planing is possible and probably not even difficult if displacement is low enough. Of course most "heavy cruisers" are not light. Most HCs are equiped w huge fresh water tanks, even larger fuel tanks, heavy propulsion equipment, large keels (many w wide flat trailing edges), frequently more than one aug. genset, heavy ground tackle, numerous household appliances and considerably heavier hulls than faster cruisers. The older GBs come to mind. Heavy for shure but w a QBBL that is compatible w higher speeds and if you want to travel at 14 knots and burn twenty some gph** ..* can do. One could easily assume that a boat that looked like a MT w about the same disp would perform about the same way. Not so or maybe not so. The CHB 34 has a straight run aft at the chine and a fairly strait run at the QBBL but at the keel the "run aft" is quite steep and very convex (curved). I think the run aft along the center line would keep the CHB from planing** ..* 18 knots? not ever. Daddyo, you should probably sell your boat and buy a planing hull. Don't worry too much about the market. You will sell cheap but if you buy fairly soon you will buy cheap too. Also you will be buying a boat that will be further down in the resession being a bigger fuel hog than your present boat. If you were clever at selling your boat and good at the purchase game you could even experience a gain in value. However, pleasure boats being a thing of passion and emotion we often think w the wrong end of our bodies when buying. Good luck if you do.

Eric Henning

Geeez!* I don't ever want to ask you if I have the right boat to come and visit you in Thorne Bay, Alaska!

It really isn't that I want a planing boat quite the contrary I and the Admiral really prefer to go slow, very slow, however the Admiral is taking less and less of a shine to anything that remotely resembles choppy/rough water and at the same time after just completing several off shore distance runs I expect in the future to see more and more big water. What I am considering is installing the ability to shorten these runs if things get testy. I would expect to run at disp speeds 98% of the time and get up and go 2%.


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Hey Dad,
No problem. The only thing displacement about your hull is the keel and depending on how you look at it the amount of power you have on board. I thought you had twin Lehmans. Don't hold me responsible for this but I'd say w 5 or 600 hp you'd be easily over 20 knots. You have a better hull for planing than the older GBs and less displacement (I think). Many later GBs have lots more power than Lehmans so compare. I'm sure you will be faster. Also I think Maeine Traders aren't that uncommon so look for somone that has more power w the same hull and compare there too. Keep that keel since you will be going slow most of the time but it should be faired off as much as possible on the trailing edge. If you really want speed I'd say go twin engines** ..* too much drag and turbulence to the infeed water to the propeller. I'd be thinkin two 300 hp Cats.

Eric Henning
Single-screw, no thruster... FTW!

I can only imagine how expensive making a single-screw into a twin-screw could be. ZOIKS! While I don't know if planing is possible, you could gain a few knots while pushing up a 6' wake with a few hundred horses under the hood.

Don't know how big Daddyo's budget is but I spoze if he was loaded he may have a 55' OA. But if he's flush and just likes his MT another option is single engine w twin screws. There was an article in PMM (feb 05) about a system of shafts and gearbox's to accomplish this and there were numerous advantages w the system. If Daddyo is poor he could get an old 400 hp engine, clean up the trailing edge of the keel and get a propeller that is better w high infeed turbulence. A 4 or 5 blade Michigan Workhorse wheel comes to mind. At any rate Daddyo better have more money than me or he won't make his MT go any faster.

Eric Henning
A repower is probably not as good as following Eric's suggestion. Sell and buy the right boat. Now what is the right boat? Depends on your wallet.

To install the horsepower to "plane" your current vessel is an easy $80K if you can reuse your fuel system, tanks etc. Plan on new/rebuilt engines, stringer upgrade, shaft, struts and props. At my Marina is the Philbrooks boat yard. They are installing the gear it systems Eric described - on current twins - using one engine. Still expensive.

For about $250 - 300K you could get a "pristine" 48 Tolly that can go slow or fast with great efficiency. Ditto a 32 NT for about $175K. Or buy a reasonably new Searay 34 for $100K. Lots of options.

-- Edited by sunchaser on Thursday 17th of September 2009 06:19:14 PM

Here is a real life example; I have a 44 C&L. Same hull as a Marine Trader, so basically your shape. Twins. I had 2 145 hp engines when I got the boat, and I could get 10knots at WOT. I added some hp, to 200 per side. Made a huge diff at low speeds, but not at WOT. I got a half a knot more. I think the hull shape is responsible, as the hole being dug at the stern just got deeper with the added power. I think 800 hp would get my boat on a plne.
Thanks for the responses guys but yes budget is everything or else I wouldn't own a 26 year old TT. As to twins I say no thanks, I'm a single screw guy till the day I drop. The lobsters boys get up and kick some butt with the same type hull so I was just wondering what it would take for me to do the same. I guess the only way to truly know as I have yet to find the same hull as ours with a big chunk of HP and compare performance is to hire a naval architect and throw money at the equation. I guess the only way to justify the change is if my engine dies and since it's a Lehman with 2600 hours on her I guess this will only be an intelectual exercise for my forseable future.
Daddy, another reason for going faster is stability. A semi-planing hull really stiffens up when it begins to plane. It is the main reason in my opnion....not so much as to limit the amount of time being exposed to the conditions. In any event, a bonus for you and your wife if you should achieve planing speeds or get a boat that does.
What size Lehman and what kind of speed are you making?

My 35' Senator with a 135 Perkins cruises at 8mph and has a max of about 9mph. If I could change ANYTHING about the old girl, it would be to be able to make about 12mph, but like you, I only have 2000 on her, so there are a lot of years left in her. And hey... AT MOST I burn 2.5 gph... usually less.
We have a 120 Lehman with a 2.5-1 reduction turning a four blade 24x22 wheel and at 1700rpm make 7.3 knts burning 2g per hr. I'd like to make 14knts or so when in a pinch.
I have a 32' semi displacement boat that cruises at 8.4 knots at 2000 rpm. She burns 4 gph at this speed. I would like to get more speed out of her since she has a Cummins 330B that delivers 315 hp at 2800 rpm. The boat's hard specs are 29.7' LWL, 16500 lbs, 24.5 X 17.5 prop, etc. Here is what Boat Diesel's prop calculator shows, although I haven't tried it yet. My goal was to get 11-12 knts at 2000 rpm.


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Length is the key for speed and fuel economy, and the reason why ships can go so fast. **For semi and full its mostly based on the length of the hull. The Eagle with a water line length of 53 ft can cruise at 10 and max at 12+ with a single 165 hp DD 671. I think, adding more HP you will dig a bigger hole.

I read somewhere that if you could push a displacement hull fast enough it would dig such a big hole it would fall back into it.* Dont know if its true, but has some merit?* *
It looks like your results and the prop calculators are quite different. To what do you attribute the difference?

-- Edited by Daddyo on Friday 18th of September 2009 11:22:41 AM
As stated in* my post....I haven't tried the prop calculator suggestion yet.
Sea horse,

Do you get full RPM at WOT and what is your speed at that stage, also how does the hull feel at that time in terms of trim and attitude? *You should be able to tell from the fuel you are burning at cruise how much horsepower you are using and get a better idea of how you are loading the engine. *Compared to the Boat Diesel spec it looks like you are low so probably not using much of the available horsepower at 2000 RPM. *

We have swapped data before, but with a 330BTA, a 2.04:1 tranny and a 4 blade 25X19 prop we have been getting between 12.3 knots (towing a heavy inflatable and full fuel and water) and 14.2 knots (lightly loaded/no dinghy) at WOT of 2,800rpm. We have been having some vibration between 1,800 and 2,000 rpm which our yard, after checking alignment, shaft run out, and having the prop reconditioned, says is probably due to using a bushing to bring the prop bore diameter down from 2" TO 1 3/4". I plan to to change the prop out for our spare which is a 24X20 4 blade with a proper 1 3/4" bore next season to see if that eliminates the vibration as the yard says the two props are essentially the same. Will see then if there are is any significant variation in rpms and speed from the current prop. Except for 5 minutes or so at WOT each time we go out, we have been running mostly at 1,600 rpm, 7.4 knots and getting around 2.6 gph.

And, no, the boat doesn't really get up on a plane at WOT. The yard's head mechanic said that at WOT she is trying to climb out of a hole that she will never climb out of. The trim tabs help but not much.

At 2,000 rpm we get around 8.5 to 9 knots.

-- Edited by dwhatty on Friday 18th of September 2009 07:00:26 PM
Once one gets over 10 knots or so a wide heavy boat acts like a plow pushing water aside and creates the hole you guys have been talking about. The stern falls into the hole, the angle of attack gets steep and just like an aircraft it takes lots of power at high angles of attack to move forward. Add trim tabs and it takes even more power but you loose some drag w a decreased angle of attack on certain hulls at certain speeds. So if you want medium speeds you've got to get a long narrow boat. Put a little rocker in the bottom and one can even go gracefully and relatively efficiently forward w a long narrow hull. Walt, I kept the techie talk out to please you.

Eric Henning
The simplest lowest cost solution , probably less than the interest on a $100,000 dollar bastardation job is a call to BOAT TRANSIT ,

about $1.50 to $2.00 per mile plus a bit of travel lift at either end.

If it keeps a 2GPH boat wholesome . instead of lugging a 40GPH engine , and destroying it thru severe under loading, and keeps da bride happy its cheap indeed.

Second choice is a simple delivery guy to bash the boat thru the rough water and a rent a car to get you there and the capt home.

Cheap & EZ.

Even cheaper,
If you look out over bow at 7 knots most of the time it looks like your'e not moving at all. Look over the side and it looks a bit better especially if one looks down. But look over the stern and it frequently looks like your'e going 15 when only doing 6. I do it all the time. I just look aft, watch the wake for a few seconds and feel like I'm making real progress.

Eric Henning
Once one gets over 10 knots or so a wide heavy boat acts like a plow pushing water aside and creates the hole you guys have been talking about. The stern falls into the hole, the angle of attack gets steep and just like an aircraft it takes lots of power at high angles of attack to move forward

Fine description on a displacement hull that's overpowered , but the wide box shape of a true plaining boat works like the wing of an aircraft , wide but narrow fore and aft is a good lifting surface , IF the boat is light enough and powered enough.

Actually it dosn't Fred. 70% of the lift on an AC wing is generated on the upper surface. And if you don't have enough power to push the wing through the air fast enough it won't fly** ..* nor will a planing hull plane without enough power to push itself out of the hole. A 35X12' boat is 420 sq ft and a boat 42X10' is the same size but the 42X10' boat will go 10 or 12 knots with much less power. With it's wider beam and higher angle of attack the 35X12' boat will make a much bigger hole to climb out of but at 10 or 12 knots it will be in the hole consuming loads of power. Medium speeds demand a narrow boat. Have you looked at the Atkin site lately? Remember the long narrow boats requireing very low levels of power? I have an 18' FG outboard and I'd like to replace it w a 26' Atkin boat (Wader) useing the same 60 hp outboard. I'd get 15 knots instead of 25 but I'd have a nice big (and long) boat. What do ya think Fred.

Eric Frederick Henning
I am using the underbody of the Atkin River Belle ,computer squezed to 7.6 beam for the test basis of our "box boat". 30LOA

The advantages of skinny boats is well known , but the efficiency of the box keel and reverse deadrise is quite hard to model test in a facility.

The numbers can only be pulled with a prop powered model , or perhaps a couple of Cray's with a specially written program.

We have a model, but without building a dozen more to compare against , it mostly is usefull for being sure our desired interior will fit.

The box keel does have a huge advantage for our use as the boat will be very EZ to get in the box with simple pipe rollers.

If you want to see a real fuel efficient boat Google the Rescue Minor , and the quite fancy version (about 20ft) built that gets 20 30 mpg at speed bu Robb White?

Yep, it is the beam on these IG's that keep them from truly planing. It also a variable that doesn't appear to be included in the prop calculator. The Mainship 35/39s have the same issues....3xxhp and still only 12ish kts.
"70% of the lift on an AC wing is generated on the upper surface."

There are about 5 theories ,why a wing works , vaccume is one.

Guess my launch will always be limited to 12K as we have had hard water on the deck, but never have been able to get a smooth flow from bow to stern over the pilot house. DARN!

There is a huge difference on boats at SL 2 or 3 , and fast boats of SL3 and way faster in terms of how the bottom (length, width) affects the pressure pattern that holds up the boat.

With a proper plaining hull , mostly weight is the determinant of the speed or efficiency.

Less IS more , if you can afford it.
Hey Fred,
Are you telling me there are millions of people flying airplanes w wings holding them up and we only have "theories" about how they work?
A year or two we talked, argued and discussed wing lift and it's possible we had considerably more than five theories.
Yes yes on the rest
Boeing must have sent Marin to mars** ..* I actually miss the guy.

Eric Henning
"Are you telling me there are millions of people flying airplanes w wings holding them up and we only have "theories" about how they work?"

Aircraft defy gravity (while they work) and there is not yet a good theory on gravity.

Or a good "Unified Field" theory that has any more proveability than "God did it!".

Just because we dont have full knowledge on a subject does not mean we cant build something.

This months PBB has a great article on a Dutch research lab , and their "new" cruising hull shape.

Sad reading for anyone looking for a "fast trawler" , even after 5000 years of boats , and a gang of Cray's working on the problem, with decades of tow tank observations.

"we only have "theories" about how they work?"

Anyone that is foolish enough to go in an Airbust may soon realize not all theories of flight will get you to a destination other than Valhalla..

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