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Old 24-01-2007, 20:08   #1
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15hp on a 3.1m dinghy?

I get the impression that a 15hp motor on a 3.1m (10') dinghy is desirable as this sort of setup should plane with a load (3-4 adults).
Problem is, the dinghys that I have looked at have been rated from 5hp(2.4m) to 8hp(3.1m).
These have been fibreglass, aluminium and inflatable.
So how does one use a 15hp motor?
Possible ideas:-
1. Just bolt on the motor and go
2. Reinforce the transom
3. Use a light motor
4. Buy a heavy, strong dinghy
Any solutions?
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Old 24-01-2007, 20:30   #2
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It could be more then just reinforcing the transom, i guess it would depend on the boat design. For example I've seen some boats with a 85 lbs motor (10 to 15hp) that one person in it has to sit in the middle or the thing will tip. I've got a 6 hp on my porta bote and I figure it's the transom that would need reinforcement. Many of the inflatables i looked at were rated for a 15hp.

Also make sure you pay attention to weght rather then hp as the 4 strokes are sometimes a bit heavier then the 2 strokes. I guess 3 or 4 are your safest options. Do you currently have one or both dinghy and motor?
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Old 24-01-2007, 20:46   #3
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I am no expert, but really, I think people often "over-spec" their outboards. I mean, if you have a yacht, your dinghy is not your primary water transport. Do you really need to be able to get up on the plane with 4 people on board? Remember that if you are going into any sort of sea-state, you are going to be hauling the dinghy and outboard on and off the boat... can you manhandle the 15hp outboard yourself? I find myself asking how small an outboard I can get away with, not how big!
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Old 24-01-2007, 20:50   #4
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Yea I agree 100% Weyalan. My 6hp is a little under 50 lbs (its old) and I can't imagine trying to get a 80lbs plus motor onto my mount (which prolly wouldnt support it as its homemade.)
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Old 25-01-2007, 01:11   #5
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Currently have a 2.6m dinghy...

At the moment I have a 2.6m aluminium dinghy with space on the cabin top for 3.1m.
It is badly dented, there is no motor, it leaks and the oars were stolen when I last slipped. No self respecting thief would take it, particularly after a few red-back spiders have taken up residence.
The boat is 44', 15 tonne and has a 2m draft so I figure there are going to be some long trips in the new dinghy.
At the moment I favour another aluminium dinghy of 2.4m, 39kg to 3.1m, 50kg.
The 15hp Tohatsu comes in at 41kg with 26kg for the 8hp so the weight should come out at 65kg or 91kg (plus extras).
Question is the bigger, faster tender worth the extra weight.
Nice looking porta bote.
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Old 25-01-2007, 07:07   #6
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Quote:
Originally Posted by Chris31415
Question is the bigger, faster tender worth the extra weight.
And the answer is ....it depends. Yes it depends on what sort of use you intend to get from your dinghy and your cruising grounds. Myself I have a 10'06" RIB with a 15 hp Yamaha 2 stroke and I wouldn't have anything smaller. For the cruising I do (Bahamas 6 months a year) your dinghy is your car, your pickup, your towtruck. The bigger the better. We go miles fishing and exploring, sometimes out in the ocean, sometimes up creeks and quite often a mile or two to do laundry, get supplies and just go into town, in not too well sheltered anchorages. If I were on a mooring and just going back and forth a small dink would be ok, or if my cruising grounds were such that I could anchor a few hundred yards from where I get ashore, again a small dink would be fine. But for my type of cruising the bigger the better. The only ones with small dinks in the Bahamas seem to be newbies or Europeans.
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Old 25-01-2007, 14:16   #7
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Vasco
Your 10' RIB is possibly rated for 8 to 10hp.
If so how does it handle the extra horses, or what power is it rated for?
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Old 25-01-2007, 14:44   #8
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Chris31415,

My AB is rated for 25 hp max. They recommend a 15hp.
AB Inflatables
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Old 25-01-2007, 20:50   #9
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We had an Avon 3.10 RIB when we cruised Mexico. It was rated for 9.9 hp, but we used a Tohatsu 15 with great success. Total weight of boat, full tank of gas and motor was around 220 lbs. We lifted it up to the caprail every night with the main halyard and the electric windlass to prevent theft.

The ratings (imho) are put there by "loooyyers". Use your common sense. 15 hp will plane four people and their stuff. It's important to have as much hp as you can drag up a rather steep beach by yourself when there's surf to deal with upon relaunch.


I still have the boat, although it's not being used as a tender. It's perfectly servicable even though it's 15 years old.
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Old 26-01-2007, 07:23   #10
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Here's one,
Clark Boats - Car Topper


Heres another on 3.5 which is my pick for my boat.

Aluminium Boats - Sea Jay Boats - Barra Punt#

They will be lively.

Dave
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Old 27-01-2007, 06:11   #11
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To determine the H.P. capacity of your boat:
First measure the length of your boat and the width of its transom. Then you multiply these numbers to get a third number, called a factor. You then use this factor to find your safe horsepower capacity.

If your factor number is between 0 and 52, your safe outboard horsepower capacity is
as shown below:
Factor = Max. Horsepower:
0 – 35 = 3 HP
36 – 39 = 5 HP
40 – 42 = 7.5 HP
43 – 45 = 10 HP
46 – 52 = 15 HP

See also:
ABYC Standard "S-12", Table I (Transom & Motor Well Standards)
Sorry, my scanner is not working

Or:
”Construction Standards for Small Vessels” ~ Transport Canada TP 1332 E
Section 4.0 - Recommended Maximum Power Calculation
Goto:
Section 4.0 - Hull Design Requirements - Construction Standards for Small Vessels (2004) | TP 1332 | Marine Safety

And:
”SAFETY STANDARDS FOR BACKYARD BOAT BUILDERS” ~ USCG
This Coast Guard pamphlet shows how to calculate your boat’s safe outboard horsepower capacity and its safe load capacity, and more.
Goto:

http://files.dnr.state.mn.us/educati...atbuilders.pdf
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Old 27-01-2007, 15:38   #12
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Ok, so 15hp might not be safe...

OK, so 15 hp on a 3.1m dinghy might not be safe.
Next question.
Under what conditions would this be dangerous?
I can understand that a flat bottomed boat cannot be turned sharply, but if one has an aluminium dinghy of conventional design what are the risks if 15hp (instead of the rated 8) is used, assuming that there are no more than 3 adults (normal rating) onboard?
i.e. What can go wrong?
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Old 27-01-2007, 15:58   #13
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Chris the M18 Tohatsu is the same weight as the 15 and the same $$$

I'm going that way and just dont use all the power unless thing's are ideal.

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Old 28-01-2007, 06:54   #14
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Potential Hazzards of Excessive Horsepower:

A larger engine may make your boat run faster, however the boat may not have been designed the handle the extra weight, speed, or stress.

SPEED/POWER:
An oversized outboard can produce more torque (and speed) than the hullform* can safely withstand, resulting in flipping. Your steering mechanism (even a hand-held tiller) may not be designed for the larger engine, leading to reduced or lost control at higher speeds.
* Flat bottom boats are generally further de-rated.

WEIGHT:
An oversized outboard can make the stern too heavy, causing squatting (transom too low to water), potentially resulting in swamping.

INSURANCE & LIABILITY:
An accident involving an “overpowered” boat, and a subsequent lawsuit, you would likely be found liable. The results of exceeding the specified capacity may be deemed a foreseeable direct or contributing factor, even if not true in specific case.

NOTWITHSTANDING:
An underpowered boat can also be dangerous, if you get caught in heavy currents, powerful seas or strong winds.
The capacity plate limits are suitable for normal operating conditions. In rough seas, bad weather or when operating in congested areas you may want to carry a lighter load.
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Old 28-01-2007, 15:20   #15
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Idealy I have had a thought in the back of my mind about having a 12 to 13 ft V nose punt were the first 3 ft was hinged, so it would go in and come out of the water as a 9 or 10 footer, but then fold down to a 12 - 13 footer with the 18 hp on the back.

I think this will be achievable, but it would be nice if someone else was the guinea pig first.

I'll need to drink some more Rum to get the imaginitive juices flowing.

Dave
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