Secondary OB for SIB...how big?

Black Rat

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Nov 22, 2018
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Two part question. I have a 14 foot SIB I use in Lake Superior around the Apostle Islands (there are 21 islands) with a 25HP four stroke. Half the time the wind and waves are such I can only do around four knots. I’m thinking a smaller engine would do me better on those days for a couple reasons. I could land the 14 footer better on the sand beaches in heavy wind waves with a light 60 pound 6HP rather than the 160 pound 25Hp on the stern. I’m leaning for the 6HP due to cost…but is it enough power in three to five foot waves into the wind?

The second question regards the aluminum seats which I have never used since I like the SIB open as much as possible. I’m thinking I could attach the 6HP to the front bow seat when I was using the big 25HP which is now going on it’s 9[SUP]th[/SUP] year so a spare would be a good idea esp in Lake Superior.

black rat lsb marina 1 (med).jpg
 

matt167

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Sep 27, 2012
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2,147
I would vote for a 100lb 20hp to replace the big one, and a prospecting small one. Selling the 25hp that you have will net you cash left over from what you wanted to spend on a 6hp.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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Personally wouldn't want nothing under 9.8 HP propped right to run at its max wot rpm range as usually loaded. Is it possible to get a 2 stroke Tohatsu, Merc in your area ? Does the 25 HP still counts with the prop factory delivered with ?

6/8/9.8 motors are usually same CC powerheads, same weight with different carbs to play between 6 and 9.8 HP. Go for the largest. A 20 will not perform same as a 25 HP it's a large and heavy 470 Sib, It's intended to be powered with at least a 30 HP motor right ?

Happy Boating
 

Black Rat

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Nov 22, 2018
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Can't get a "new" two stroke up here. I did change out the original prop on the 25HP for a power prop versus speed prop and it helped a lot to get up on plane with loads. The 14 footer will take a 40HP legally but the 25HP seems to do fine on it with 4 people, gets right up on plane. They have a Tohatsu 9.8HP at 82 pounds and I guess I'd rather have a little more HP esp with five footers and wind if I was in a pinch trying to get on the leeward side of an island in the Apostles. Dang weather can go from calm to six footers in 20 mins up there. Pic of "Black Rat" in 3 footers, the worst this boat has encountered thus far in two seasons...

black rat 3 footers a.jpg
 

Sea Rider

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So the prop was changed, which diam/pitch was the original one factory delivered with said motor ? was changed to which other diam/pitch ?

In those kinds of unpredictable water situations can't possibly plane a Sib for long time periods unless having steel arse and kidneys LOL!! Much better is to throttle to a position to achieve fast displacement speed while maintaining the Sib riding parallel to water level at a slow pace. Will definitely be a boring return taking you back safe to terra firme.

You plan using just a single motor depending on the water cond, right ? If going for the Tohatsu will need to change current prop for at least one with less pitch from factory delivered to rise rpm and respond quickly when more throttle is in demand under more load and wind while being powered with a 9.8 HP motor used as main.



Happy Boating
 
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Black Rat

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Nov 22, 2018
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I'd be hard pressed to tell you the diameter/pitch of the props. All I know is the original prop gave me more speed but longer to get it up on plane. The new prop pops me right up on plane but max speed knocked down from about 30 knots to about 23 knots? I've included a pic of the two props together and some literature with the original prop but honestly don't know which is which. I'm guessing the prop on left is the secondary one I bought and gives me more instant power?

mercury prop 25hp 2.jpg

And yes I would take the 25hp off once the waves get too big and use the 9.8hp since I don't need the power and have to go slow anyway.
 

matt167

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The 9.8hp Tohatsu is the same engine as the 15hp just de tuned, so you might as well get the 15hp
 

Sea Rider

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The 9.8hp Tohatsu is the same engine as the 15hp just de tuned, so you might as well get the 15hp

NOPE, the 6-8-9.8 are 262 CC alike, the 9.9-15 are different 333 CC water animals. Sometimes boaters mixes up 9.8 with 9.9 HP motors...

Happy Boating
 

Sea Rider

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Black Rat,

If portability, lighter weight and price is secondary and better water performance is more preferred, why don't you go straight for a Tohatsu 15 HP motor ?

Will tell you by plain experience installing portable tiller motors that no matter which brand they are if being factory delivered with a prop it's definitely with a medium pitch one for manufacturer to play safe as won't know in which boat type will be installed.

The delivered prop is a higher pitch one , usually has a lousy hole shot, worse with more load on deck, but better top end speed while probably revving somewhere min to middle wot revs factory stated.

The new one has a less pitch number, has better hole shot with more load but will lose top end speed while probably revving middle to max wot rpm range. Can't have both top situations in a single prop, it's one or the other. For me, top hole shot is mandatory as never run at full grip all the time while motor swallowis unnecessary galons of fuel to run 1-1.5 MPH faster at the most with a Tohatsu 18 HP.

Will highly recomment to buy an induction Hardline hour/tach installal it and check max wot rpm the motor achieves while running fully loaded on flat, calm water conditions. Will serve you well if plan going for the 9.8 motor and maximize a less pitch prop to power much better the Sib while encountering tall waves and harsh wind.

The 25 HP motor is a 2-3 cylinder one ? Just for the record, check the stamped numbers on each prop body and report numbers...

Just curious, you've been here posting at the forums before ? If, so , welcome back!!

Happy Boating
 
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Sea Eagle Mark

Seaman Apprentice
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Feb 4, 2019
Messages
40
Hey Black Rat,
Sea Rider had some great advice on the props. However, I'm a Tohatsu outboard fan and his insites on their specifications were a little off..... Here's the correct information on a few of their motors gotten from their website. Sea Rider is correct suggesting you get the biggest motor you can afford and handle and then re-prop it to handle the conditions you'll most encounter.
IMHO if weight and price are your main concern I'd go with the 9.8. If you can handle a little more weight and your pockets are a little deeper go for the 15 or 20 hp.

Tohatsu outboard specifications.
​​​​
6hp 123cc 1 cylinder
Weight 55 lbs

8hp 209cc 2 cylinders
Weight 81.5 lbs

9.8hp 209cc 2 cylinders
Weight 81.5 lbs

9.9hp 333cc 2 cylinders
Weight 94.7 lbs

15hp 333cc 2 cylinders
Weight 94.7

20hp 333cc 2 cylinders
Weight 94.7
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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10,689
SEM,

There's 2 four stroke 6 HP motors available :

-MFS6C- 1 cylinder / 123 CC / 25 Kilos (Old model

-MFS6BZ - 2 cylinder 209 CC / 37 kilos (New model)

So 6BZ-8-9.8 with same CC and weight, right ?

Other nice alternative depending on OP's budget will be a 9.9 or 15 HP motor which weights exact same but heavier than a 9.8 HP.

Besides being a huge fan and user of Tohatsu motors for years now, I also sell and install them, have 3 in permanent demo use all of which have gone through a prop maximization which is a day/night performance difference.

Happy Boating
 
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