25-Mph Lake Cruiser - New Boat Build

comstox

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Guys I am wanting to discuss a possible new boat build, and I need some sage advice! I don't want to waste time and resources going down the wrong path.
We have a 1969 Side-Winder Lo-Pro with a 75hp Merc (It's my Avatar pic on my profile!) We use it to cruise around our beautiful New Hampshire lakes. (We certainly have a lot of beautiful lakes around here!) But that beast EATS FUEL! Cruising around the lake at 20 - 30 Mph can easily suck down two 6 gallon tanks in a day!
I want to take a 20?30 hp engine and maybe a small John boat or something like those Jet Dingy sized boats and make a two person lake cruiser.
It must be stable and maneuverable and plane up to at least 25mph.
Can you guys weigh in on this topic??? Maybe add pics of similar little boats. You guys have a wealth of information floating around in your collective craniums and I long to hear what you guys have to say!

P.S. I don't want the boat to be at FULL THROTTLE ready to explode to achieve 20-30 mph: Is that asking too much?


Thanks in advance!
Glen in New Hampshire
 

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Scott Danforth

Grumpy Vintage Moderator still playing with boats
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boating costs money

if you think your boat sucks fuel, a Jet dinghy uses about 3 times the fuel since a jet is so much less efficient than a prop. Jetskis will easily burn 20-30 gallons per hour

your 2 cycle motor is burning 0.55# of fuel per hp per hr
a 4-stroke would burn about 0.40# of fuel per hp per hr
a diesel will burn 0.33# of fuel per hp per hr

so your fuel burn is directly dependent on how much power you need to move your boat in a given current, wind, load, speed, etc.

back off your RPM so your boat is about 2-3 mph just above planing speed. this is probably your most economical planing speed
idle your boat along is the most economical speed.
 

comstox

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boating costs money if you think your boat sucks fuel, a Jet dinghy uses about 3 times the fuel since a jet is so much less efficient than a prop. Jetskis will easily burn 20-30 gallons per hour.

YIKES!!! That is good information to know. I should have specified that I don't want a jet boat, but a small style boat like that.
I like what you said about just backing off the throttle a bit after reaching plane.
Thank you!
 

Scott Danforth

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BTW, nice looking little run-about you have there.

is it the fact that you burn up 12 gallons of fuel, or is it the pain in the butt to pull the tanks and fill them up? if so, simply look into a gas caddy.

my averaged outing for fuel is about $100. sometimes more. then again, I cruise about 80 miles on average outing (its 13.7 miles from the boat ramp to the ICW)
 

zool

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A 13 whaler with a 25hp should top out around 30mph and get around 10+ mpg at 25mph cruise,,,IIRC
 

comstox

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is it the fact that you burn up 12 gallons of fuel, or is it the pain in the butt to pull the tanks and fill them up?

No it's not about the pain in the butt, It's a money and principle thing; I just don't feel I should burn that much fuel. Or at least I don't WANT TO burn that much. :lol: I'm actually thinking of putting the 75hp Merc on a bigger boat and putting a newly acquired 50 Classic in it's place. - Any thoughts on that idea Scott?
Thanks for the response!
 
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comstox

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A 13 whaler with a 25hp should top out around 30mph

DO you feel a 10' v-hull aluminum boat would do the same? and that I could possibly get by with a 20hp motor? I am planning on enclosing the bow and attaching a low profile windshield. Thoughts???
Thank you!
 

zool

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DO you feel a 10' v-hull aluminum boat would do the same? and that I could possibly get by with a 20hp motor? I am planning on enclosing the bow and attaching a low profile windshield. Thoughts???
Thank you!


For me it would be a matter of stability, I have a 10' tinner and its barely stable with one adult.....
 

comstox

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For me it would be a matter of stability, I have a 10' tinner and its barely stable with one adult.....

Now remember... I have to convince my wife to get in this boat with me!
How about this pink boat??? :lol::lol::lol::lol::lol:
 

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Scott Danforth

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No it's not about the pain in the butt, It's a money and principle thing; I just don't feel I should burn that much fuel. Or at least I don't WANT TO burn that much. :lol: I'm actually thinking of putting the 75hp Merc on a bigger boat and putting a newly acquired 50 Classic in it's place. - Any thoughts on that idea Scott?
Thanks for the response!

if you are looking for economy in a boat, get one with oars, sails or paddles.

12 gallons of fuel per outing is nothing in the grand scheme of things.

50hp vs 75 hp will burn the exact same fuel just tooling around on plane. both 2-strokes burn the same amount of fuel per hp. 0.55# of fuel per hp per hour.

the 75 will burn more if you scream along at WOT.
 

Scott Danforth

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DO you feel a 10' v-hull aluminum boat would do the same? and that I could possibly get by with a 20hp motor? I am planning on enclosing the bow and attaching a low profile windshield. Thoughts???
Thank you!

maybe you should look into a early 70's wooster hellion. 7' long, powered by a 9.8, does 25mph. then you are only burning 10hp worth of fuel.

then again, if you idle your 75hp motor along, only commanding 10hp, you would burn the same amount of fuel as a 10hp motor at WOT.
 

comstox

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you would burn the same amount of fuel as a 10hp motor at WOT.
Scott... I'm a total boat noob. Just got out first motorboat (that Sidewinder Lo-Pro last year.) You have helped me tremendously and I'm now beginning to understand how the whole HP to fuel ratio thing works.
Also, what in God's green earth does WOT mean??? remember... noob here.
Thanks again.
 

comstox

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I looked up the Hellion. I don't think I'd ever find one available... But maybe. I'm talking about a small boat like this picture. Something Cool, Zippy & Stable!
Is there a ection on the forum for "little boats?"
 

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Scott Danforth

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Scott... I'm a total boat noob. Just got out first motorboat (that Sidewinder Lo-Pro last year.) You have helped me tremendously and I'm now beginning to understand how the whole HP to fuel ratio thing works.
Also, what in God's green earth does WOT mean??? remember... noob here.
Thanks again.

Wide Open Throttle.

your motor only uses as much fuel as you command. if you want to go fast, it will cost you fuel. if you want to conserve fuel, simply idle along.

boats are extremely inefficient. Jet boats are worse as a prop is about 3X as efficient as a a jet.

your boat is most likely getting between 2 and 3 mpg on a good day. so with your tooling around in your boat, you probably put between 24 and 36 miles on the boat.

again, 12 hours of fuel burn per day is nothing.

to put it in perspective, I have been on yachts that burn 13.2 gallons of fuel per second (the budgeted fuel consumption is $150k USD per day) so spending $30 per day or even $100 per day to have a face full of smiles is worth it.
 

comstox

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I have been on yachts that burn 13.2 gallons of fuel per second (the budgeted fuel consumption is $150k USD per day) so spending $30 per day or even $100 per day to have a face full of smiles is worth it.

Say WHAT???? :eek::eek::eek:
WOT I get it!
Thanks again!
 

Bayou Dave

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When I got my boat I found a fuel burn chart for it. I learned that running it at 4000 rpm's was the sweet spot. That keeps me on plane and it uses a little over 5 gal per hour. Find your sweet spot and you will cruise right along and reduce fuel usage.

2002 Suzuki DF140 4 Stroke
EngineSpeedFuelRange(a)
(rpm)(mph)(gph)/(mpg)(miles)
10004.40.3/14.7648
15005.80.9/6.4282
20006.81.4/4.9216
250010.02.4/4.2185
300011.43.2/3.6158
350018.04.1/4.4194
4000(b)25.65.3/4.8211
450027.65.9/4.7207
500033.77.4/4.6202
550035.19.1/3.9171
600039.811.2/3.6158
6200 (WOT)41.711.8/3.5154
(a)Based on 90% fuel capacity
(b)Optimum cruising speed
 

comstox

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When I got my boat I found a fuel burn chart for it. I learned that running it at 4000 rpm's was the sweet spot. That keeps me on plane and it uses a little over 5 gal per hour. Find your sweet spot and you will cruise right along and reduce fuel usage.
Mine is a 1986 75 HP Mercury 4 cyl - 2 stroke. Where do you suppose I could find a burn chart for it?
Thanks!
 
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Bayou Dave

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You might find it in a repair manual. That is where I found mine. Or do a search for your motor and fuel usage chart.
 

comstox

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Well will you look at what I found! a 10' GW Invader! It's 10 feet from stem to stern! and rated for up to a 55HP engine. I think it's PERFECT! $1,000 with no engine. Do you think it's a good buy?
 

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