How fast ......??

[jasper2447]

Hi everyone

After quite an absence from the water, I have now got a boat again - just one year old.

I took it out last weekend and felt a little disappointed by its performance and one or two niggly faults, so I gave it a couple of hours attention this evening.

On Sunday at WOT I was only getting 15 mph - verified by both wheel type speedo and GPS. I was expecting more like 20 +

Anyway this evening I found that the throttle and gearshift cables were well out of adjustment and set everything up again. I might get a chance to launch her again this weekend and want to know what speed I might expect to get, all things considered.

DETAILS - boat is a 16 foot Seajeep by Seahog Boats. It weighs only 350 Kgs (770 lbs) withoutthe engine.
The engine is a Mariner 25 HP with electric start and full remotes
With two people up and two full fuel tanks 15 mph seemed very slow and I'm not now certain that I was actually at WOT

QUESTION .. Is there a rough formula for how fast I ought to go given the boats weight, length and the engine' horsepower?

Thanks in anticipation

John B

 

[JB]

Re: How fast ......??

Yes, John. There is a formula for estimating potential top speed.

MPH=(square root of HP/Wt)X bottom factor.

HP = Horsepower
Wt= total weight of the vessel in pounds

Bottom factor: Deep vee= 150, Semi vee=200, flat=225

If you estimate the total weight at 1200LB and the bottom of your boat as a deep vee, then. . .
25HP/1200=.0208
Square root of .0208=.144
X 150=21.6MPH

If your boats deadrise is low near the stern you might bet a bit more if prop, balance and trim are optimized.

 

[Silvertip]

Re: How fast ......??

Or use any one of the on-line prop calculators. You need to know the wide open throttle rpm of the engine, prop pitch, gear ratio, and slippage. For slippage, use 12% as a nominal figure. It should get you in the ball park within a couple miles per hour.

 

[jasper2447]

Re: How fast ......??

Thanks JB - that was a fast reply and a super formula. i'm sure a lot of members of the forum will find it useful

Actually my boat is a cathedral hull, so hopefully the bottom factor of 200 will apply, giving me atheoretical top speed of more like what I expected , say 25 - 28 mph

Many thanks again

John

 

[tashasdaddy]

Re: How fast ......??

you are expecting alot from a 25 hp on a 16 ft. cathedral hull.

 

[jtexas]

Re: How fast ......??

maybe 28 KPH?

 

[Dakota47]

Re: How fast ......??

my 16ft tri hull thundercraft

with a 60hp merc would do 30 mph max..

 

[jasper2447]

Re: How fast ......??

Great JB - many thanks for that

It's actually a cathedral hull, so i guess the bottom factor of 200 would apply, making the top speed more like what I hoped for, i.e. between 25 and 28 mph

That's a great formula

Thanks again

john

 

[tommays]

Re: How fast ......??

Looking at your photo the high cabin has much more wind resistance than any American brand of boat that size

When you only dealing with 25 HP it is a big factor in the formula that is missing


Tommays

 

[rndn]

Re: How fast ......??

Thanks for the speed calculator it worked exactly for my boat.

 

[jasper2447]

Re: How fast ......??

I guess that's true, but it's what makes the Seahogs special - designed by fishermen for fishermen, not speed freaks. That forward cuddy is just great and its height not only gives comfortable seating inside, but it removes the need for a windscreen. and that big central hatch makes recovering an anchor very, very safe - no need to stand on a pitching foredeck straining on the anchor rope while trying to balance !

It's all part of the compromise I was talking about earlier - cabin size vs deck space, roof height vs speed and engine size vs fuel economy.

You'll see from the attached photo that she has really unusual lines, but the Saahogs are becoming one of the most popular (and most copied) designs in the UK. have a look at their website for the full range ... seahog.co.uk/Boats.htm
John

 

[pootnic]

Re: How fast ......??

hope im not butting in..i have a 360# V series jon with a 40hp..using the formula i should be cruising around 50mph...gotta get a gps,i hope im not going that fast

 

[JB]

Re: How fast ......??

I think you have maybe neglected a lot of weight beyond the bare hull, pootnic. Total weight includes passengers, engine, fuel, battery, tackle, electronics, etc, etc.

 

[BF]

Re: How fast ......??

Hi there,

ditto what's been said above... I think that's a lot of boat for a 25 hp. I think you're doing well to get it planing. On their website I saw that it's rated for up to 50 hp... IMHO 50 hp would be WAY better. That'd probably get it in the 30 mph neighbourhood. With 25 hp, I'd be concerned that it wouldn't have enough power to get it on plane with much weight in it. If you ever took a wave over the back or otherwise took on water, with a 25 you might not be able to move it fast enough to 'power bail' it.

With my past experience with our family trihull in the 70's/80's (my teeth are still rattling) having a motor with power tilt/trim might be nice too. That way you could adjust the trim to try and soften the ride a bit. Ours was a 15' runabout that more or less was comparable hull wise, but had the traditional windshield sleeper seat arrangement. It came with a 40 hp on it, but once we started skiing behind it more, we upgraded to a 60 hp. It would do about 33/34 mph with 2 people.

Don't get me wrong... I'm sure you'll still catch a load of fish with your boat and 25 hp, but I think the boat would behave better with more hp... Of course I'm presuming you WANT a planing hull. Maybe you're content to putt around at displacement speeds in which case you could get by with even less than the 25 hp.

Anyway, that's my $.02... Good luck fishing.

 

[pwiseman]

Re: How fast ......??

Yes, John. There is a formula for estimating potential top speed.

MPH=(square root of HP/Wt)X bottom factor.

HP = Horsepower
Wt= total weight of the vessel in pounds

Bottom factor: Deep vee= 150, Semi vee=200, flat=225

If you estimate the total weight at 1200LB and the bottom of your boat as a deep vee, then. . .
25HP/1200=.0208
Square root of .0208=.144
X 150=21.6MPH

If your boats deadrise is low near the stern you might bet a bit more if prop, balance and trim are optimized.

This is very close for my boat with a 30 HP Honda. Boat is of similar weight to the one being discussed. 660 hull, 185 lb motor + gear and flesh. With just me, I got the thing up to 24.5 running a 12 pitch oem honda prop. With 3 peeps, I drop to a 10 pitch and it gets about 21. This was neccesary to get the boat on plane. I used GPS for measuring speed.

BTW, I just traded the honda 30 for a Merc 40 EFI as I felt I can use the extra pop. My boat is a lund Rebel 1450 (deep v aluminum). Went merc to get away from carbs and only Suzuki and Merc have a 40 HP EFI at the moment. The merc is about 30 lbs lighter then the suzuki and I previously had a 50 Merc EFI that was very solid, but too heavy for the boat I had it on.

 

[mrfixitman40]

Re: How fast ......??

wow just tried that formula and it is pretty darn close thanks for the info i am gonna keep that in mind