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©2005
Tsunami Fishing Systems
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How Much Power Do You Really Need?
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One big question at the start of the search was: "How big a
hull can I push with my old motor?" I wanted to go with as large a
hull as possible, in fiberglass if possible, but on the grand scale of things,
90 horsepower is not a lot of juice.
In the case of Toy Boat 2, I would be trading off speed for size. Here's
how I was predicting things would wind up weight-wise:
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| Item |
Toy Boat 1 |
Future Toy Boat 2 |
| Motor
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305 lbs
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305 lbs
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| Hull
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750 lbs
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1,200 - 1,750 lbs
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| Gas tank / Oil reservoir
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13 gallons / 1 gallon
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35 - 50 gallons / 1 gallon
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| Batteries
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2 x Group 27, 1 x Group 24, wet cell
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2 x Group 27, 1 x Group 24, wet cell
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| Boat gear (anchors, etc.)
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50 lbs
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70 lbs
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| Auxilliary power
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41 lb thrust Motorguide 12V
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65 - 80 lb thrust 24V electric
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| Livewell / Bait tank
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18 gallon
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30 - 45 gallons
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| Food & drink, ice
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12 lbs
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35 lbs
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| Passengers & personal gear
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2 @ 200 lb
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2 @ 200 lb
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| Fishing tackle
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35 lbs
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35 - 50 lbs
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| Estimated total
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1,950 lbs |
2,700 - 3,400 lbs |
Would the old 90 HP be enough? Gut feel says "No". However, a couple
of reference books ("Fishing for Tuna and Marlin" by Capt. Peter Barrett, and
"The Fisherman's Boating Book" by Bob Stearns) offered the following
rule-of-thumb formulas for monohulls:
For a modified-V hull (I'm assuming 16 degrees or less):
Total Weight / 30 = HP required to drive the boat at 25 mph, at a 75%
throttle setting
For a deep-V hull:
Total Weight / 25 = HP required to drive the boat at 25 mph, at a 75%
throttle setting
Using these formulas, Toy Boat 2 would need somewhere between 90 - 136
HP. My experience has been that under most conditions, a cruising speed of
20 mph is actually more realistic offshore, due to the effects of wind chop and
swell. So a 90 HP motor looked to be just about right. Remember, these are
general rules, and I wasn't expecting them to be more accurate than
perhaps +/- 10%.
Incidentally, we have checked these formulas on a number
of published boat tests, and they generally seem to hold, when you allow
for variations in horsepower
(see sidebar).
We've also incorporated this formula into
a calculator in our "OSJ Services" section. Please take a look
and give it a try!
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