Engine requirement

[arcadiainc]

Recently I purchased an 18 ft. deep vee lapstrake constructed of fiberglass. It is an old boat in excellent shape. The boat came powered with a 225 hp Evinrude. Grossly overpowered. The boat has no hull id or Coast Guard plate. Using the Coast Guard hp calculation formula the boat should have a 115 hp engine. I have a 70 hp Mercury I am not using. An excellent motor. The boat is not heavily built. Is there a possibility I can mate the two and get reasonable performance? Thank you in advance.

[Joakim]

I have no idea what kind of a boat you are actually talking about, but here in Finland 70 hp would be just fine for most 18" boats and they would have a top speed around 30 kn. How much does it weigh? How deep is the V? What is the top speed with 225 hp and how much do you need?

Joakim

[arcadiainc]

Thank you for the reply Joakim. The boat weight (a guess) is 320 kilos. The the angle of degrees at the transon is 30 (measured). The top speed with the 225 hp motor approaches 50 knots (gps). The boat is fast. My speed requirements with a 300 kilo load is no more than 25 knots. The wife and I want to cruise the intercoastal water way in the south east US.

[marshmat]

Hi arcadiainc,

Your expectations of being able to run a bit short of 25 knots with 70 hp sound fairly reasonable. Given that you're reaching about 50 knots on 225 hp now, and after running a quick (and very rough) Savitsky estimate, 20 to 25 knots on 70 hp at ~650 kg displacement seems to be in the right ballpark.

From what I've heard of the ICW (I've never cruised it), there aren't many places where you'd be able to open it up to 50 knots anyway. A deep-V at low planing speeds isn't a very efficient shape, and does make a fairly big wake, but I tend to agree that you'll find it a more civilized, more manageable (and much cheaper to run) boat with a 70-horse outboard than a big 225.

[arcadiainc]

Matt---Thank you for your reply. I made a mistake in my earlier reply. The angle of degrees at the transom is 20 not 30. A typo and I did not proof well enough. The fuel economy is of interest to me. My primary fuel tank is 20 gallons. The Intercoastal is not a hotbed of marinas. Fuel is difficult to get in places. The 225 gives me 2-3 mpg. The little Mercury does much better. I very much appreciate the information. I have to purchase new control cables etc. I did not want this expense to find the engine is too small. I bought the boat to use in the Gulf of Mexico. Some places the water is rough. The deep vee takes it in stride.

[Joakim]

Originally Posted by arcadiainc

Thank you for the reply Joakim. The boat weight (a guess) is 320 kilos. The the angle of degrees at the transon is 30 (measured). The top speed with the 225 hp motor approaches 50 knots (gps). The boat is fast. My speed requirements with a 300 kilo load is no more than 25 knots. The wife and I want to cruise the intercoastal water way in the south east US.

As a rough estimate the speed of a planning vessel depends on SQRT(POWER), thus you should get 50*SQRT(70/225)= 28 kn, which is close to what Savitsky predicts (with corrected V angle).

The fuel consumption depends mainly on the power demand, which doesn't change much by changing the motor, if the cruising speed is the same. But your 225 hp motor is probably old tech, which has low efficiency, especially on low partial load. If it was a 4 stroke or a modern 2 stroke motor, you would not have a big difference in fuel consumption.

I couldn't find any results with such a big difference, but here is one comparing 150 and 225 hp:
http://www.buster.fi/fileadmin/user_...tfacts2007.pdf

Not the lesser consumption of 225 hp motor at 36-40 kn speeds and only a bit more at 20-33 kn.

Also note that this is a much bigger boat and still consumes clearly under 1 l/NM, which translates to over 4 mpg (is mpg for nautical miles???).

Joakim

[tom28571]

I would want to run the boat with the current engine with the load adjusted to represent a typical displacement and CG with the 70 hp. Note the speed at which it is comfortable planing or below which you would not want to run because of bad attitude and/or wake.

The window between that speed and the top projected speed with the 70 may not be wide enough to make it a practical set up with the 70. I ran one test on a boat for a prospective buyer where the minimum planing speed was 20 to 21 mph and the top speed was about 25 to 26 mph. No one would be happy with such a boat and I advised him to run away from it. At any speed between hull speed of 5 mph or so and full planing a deep V of 20 degrees is going to be an unhappy boat to drive.

[Joakim]

The Buster XL has a 19,5 degree V, weighs 591 kg without engine and with two on board has a top speed of 27,5 kn with Yamaha F60 and 33,5 with Yamaha F80. Unfortunately this is in Swedish, but you can find the facts from the tables:
http://www.buster.fi/fileadmin/user_..._Sv_buster.pdf

Your boat is much lighter, but also much older design. I would believe 70 is enough.

Joakim

[Lt. Holden]

Years ago, in Florida, my family had a 19' Sportcraft Fiberglass lapstrake boat with a 75 HP Chrysler on it. It was a very nice family boat; accomodating 6 people, towing waterskiers, diving and fishing etc. I don't see how you could go wrong with the 70 HP Merc.

[arcadiainc]

Thank each and everyone for your replies. Obviously much thought was put in your answers. I had considered using the 70 hp Mercury several times. However each time I concluded it was too small. I now have a plan of action. The boat will be weighed. If my guess concerning the weight is relatively accurate I will proceed with the repower. I had the Mercury on a heavily built 15.5 foot bass boat. The boat would gps 37 mph. Upon futher reflection I feel possibly the bass boat is comparable in weight to the lapstrake. The lapstrake has a much larger profile however. Again thank you. I will post the results.

[tunnels]

225 on a 18 foot boat is a little over the top but remember you dont have to push the throttle all the way down .
I have a 14.6 with a 115 yamaha stepped transom raised the motor and it goes like crazy with a 21 p x 12,7 dia prop ,its a pleasure to drive at 2500 to 3200 rpms as the motor is just cruising and will last forever only using a fraction of it capabilities . It trolls well at idle all day long if need be !.
With 4 adults it can handle beautifly without hardly noticing the extra weight !

Under power and you have the throttle down all the time and the gas bill keeps going up as well as the repair bill of a over stressed motor .
18 footer does well with a 150hp to 175 hp motor

[Lt. Holden]

While I don't dispute tunnels numbers, I am pretty sure that the 225 hp is way too heavy for your boat. It doesn't matter whether you go full throttle or not, you are hanging way too much weight on a transom that couldn't possibly be designed to accomodate it. Add to that the fact that you are carrying several hundred pounds of excess weight which will have a significant impact on performance, handling (likely porpoising) and fuel economy.

As you mentioned you should first weigh the boat and then measure the pertinent hull dimensions (for a Savitsky calculation) and go from there. I think you will find something satisfactory in the 70 to 115 hp range. Since you indicated a desired cruise speed of 25 knots or so you should keep that in mind. As someone else mentioned you also need an additional margin of performance to accomodate loading, as well as sea and wind conditions.

I grew up in Miami and we cruised the ICW frequently. As you accurately pointed out you can seldom get up on plane (due to the no wake zones) so low speed smoothness, quiet operation and economy should be important considerations.

[designer4life]

From my experience 225hp is over kill! Keep in mind speed is not all about power but also how much is the hull of the watercraft willing to plan. Some hull designs are ment go faster than others. however in your case a 70hp would be fine. i have ran multiple watercrafts with that size engine and it seems that many times the horse power put on the water craft is just over kill.