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Old 18 May 2019, 22:35   #1
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10'8" SIB with 5hp Lehr

I'm new to the boat world(and here)and have a few questions concerning motor height and size. I took the SIB and motor out for it's maiden voyage and was a little surprised with the amount of turbulence the motor was making and I'm not sure what the correct flow should look like. I realize that the boat is light weight and the motor may/may not be the wrong HP even though the specs say 15hp max. After doing some reading, it seems that prop size and pitch make a difference, too. The prop is a 7 1/2" X 7". Could this be causing some of the turbulence? Also. is their a rule of thumb for how much the prop should in the water... such as location from cavitation plate to the bottom of the transom. I believe mine too low in the water. If so, I need to raise the transom and are their recommendations to make or buy hardware for raising it?

BTW, the boat is a hydro-force(from Costco)rigid aluminum floor with inflatable keel

Thanks, Brian
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Old 19 May 2019, 01:46   #2
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First of measure the height of the transom on the back of the boat.... Ie from the top where the motor mounts to the bottom at the centre. If this is about 15 inches you need a standard (also called short) shaft engine. This should measure somewhere between 15 - 17 inches from the top of the transom (with the motor on the boat and dropped fully down so the top of the mounting bracket rests on the top of the transom to the cavitation plate which is the plate just above the propeller.

Then put the motor in trim position 1 or 2 ie trimmed in most of the way.

If you set up like this it should be good to go.

Perhaps you could post some pics of how yours is mounted at the moment
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Old 20 May 2019, 10:56   #3
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Post pictures of your set up.
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Old 20 May 2019, 11:32   #4
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Welcome to the forum migraine.

Can you explain what you mean as turbulence?

Are you one up and just getting the boat on the plane or doesn't it plane even at maximum throttle.

Inflatable boats for their size do make a lot of wash... particularly when at/near maximum displacement speed.
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Old 20 May 2019, 14:34   #5
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IT sounds like for what ever reason you are at max revs with a big boil of water behind you which you will get until the boat jumps over the bow wave and gets on the plane I think you will struggle to get on the plane with the 5 hp but try getting as far forward as possible with your body to get the weight distribution better. The lehr engine is a propane engine I believe not sure if it competes with a petrol engine for performance someone will know
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Old 21 May 2019, 01:18   #6
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5 hp should be enough for a boat that size to proceed at a decent speed, although it may only get up on the plane in perfect conditions, with one person on board, sitting well forward. (I used to be able to plane a similar sized boat with 3 hp, but only as an exercise to prove it could be done. It was not sustainable.)

The exact height of the motor on the transom matters if you are going very fast. With that set up, I'd expect you to be poddling about at immersed hull speeds and it won't be critical.

The two things that are easiest to address are:

1) The angle of the motor. There is usually some sort of adjustable stop, so that the engine can be vertical, or tipped slightly forward or backwards. I find on my boat that one stop difference can have an enormous effect on performance.

2) The balance of the boat. It is tempting to put the pilot and all the gear well back in the wide and easily accessible part of the boat. However, that puts the stern down and the bow up. Get some of the weight forward. Consider getting a tiller extension so you can sit further forward.

Remember that at immersed speeds (not planing) the nearer you get to maximum speed, the more water you are pushing. Back off the throttle a bit for a quieter, more comfortable, more economical ride.

As for adjusting the height of the motor on the transom: the manufacturers knew the standard engine sizes when they decided how bg to make the transom. You might get some slight improvement by putting a spacer to raise the engine an inch or so, but I'd leave that until last.

I see a boat of this size with 5 hp as a nice capable craft for exploring rivers and coves, but I don't see it going everywhere on the plane, which it might achieve with a 9.9hp motor.
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Old 21 May 2019, 03:54   #7
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engine height info
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Old 21 May 2019, 14:23   #8
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Thanks for everyone replies. Most of the weight is near stern(me, motor, and 5gal propane bottle). I'll look into a getting a longer hose. The top of the cavitation plate is 1/5" below the bottom of the transom. Raising it will put the mounting bolts right at the top of the existing transom. First trip was me and the 50lb dog. Next one will be with the wife and maybe the dog, too. She's 130lbs and will be sitting forward so that may help. I do wonder about the prop being the ideal size for this boat/motor combo. I should have gone with a larger motor but weight was an issue with my bad back. I do see that this will never be a ocean/bay boat with the motor I have :~(

The wife was looking forward to crabbing.
Thanks again, Brian
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Old 21 May 2019, 14:30   #9
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It is rare a 5hp motor needs any prop change from standard.

Brian to help we do really need to know what you describe as turbulence. Can you Google an image of a boat wake that looks similar to what you're experiencing if you don't have an image of yours??
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Old 21 May 2019, 14:54   #10
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Fenlanders right as much info as possible lots of expeariance on here to help, all have been there but something I will say it's not all about flying about I spend many an hour at at 5 knots I want to see the area my boat will 22 knots but it's no good to me all the time missing stuff because I am consecrating on the next wave if you understand my thoughts ok your low on HP but it's not the end of the world IMO
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Old 21 May 2019, 16:03   #11
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Quote:
Originally Posted by migraine View Post
I should have gone with a larger motor but weight was an issue with my bad back. I do see that this will never be a ocean/bay boat with the motor I have :~(
As regular readers may be tired of me saying, I used to do long days on 3 hp on a boat of about 3.4 metres, with diving gear on board. A long day might be 24 miles on the sea or sea lochs on the west of Scotland. People sail slow moving dinghies in such conditions. They kayak in such conditions.

Speed is good for three things
  1. Getting there quickly, when you have a definite objective such as a dive site.
  2. Getting back quickly when you're tired, or the weather is changing, or you've misjudged the time.
  3. Going fast for the exhilaration of going fast.

Going slowly is good for:
  • Being comfortable when it's choppy.
  • Staying dry.
  • Avoiding or reducing wind chill.
  • Not being antisocial in crowded water.
  • Watching the wildlife.
  • Being relaxed and pensive, reading a book while someone else steers, or towing a fishing lure.
  • Exploring narrow rivers, coves, small bays, sea caves, gaps between islets.
  • Saving weight, fuel, cost and effort.

A Ferrari in a traffic jam is an expensive noisy car doing the same speed as everyone else, but with a driver who is more frustrated. A 25 kt powerboat chugging through moorings, or trickling through narrow gaps, or cruising up the river where there is a 6 kt speed limit in force, is more frustrating than the cheeky little boat bimbling along at 1/2 or 2/3 throttle at exactly the same speed.
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Old 21 May 2019, 17:12   #12
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I meant to say that the cavitation plate is 1.5" below the transom bottom
I am hoping to take it out this weekend with the wife so I will try to take a few pictures with motor running at hi, med, low RPMs
Going to play with the fishfinder, too.

-Brian
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Old 21 May 2019, 17:29   #13
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Quote:
Originally Posted by migraine View Post
I meant to say that the cavitation plate is 1.5" below the transom bottom
I am hoping to take it out this weekend with the wife so I will try to take a few pictures with motor running at hi, med, low RPMs
Going to play with the fishfinder, too.

-Brian
I'd lift that as far as practically possible see if that improves things. Running deep creates more drag which hurts efficiency
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Old 26 May 2019, 21:59   #14
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Motor/Transom Height Issues.-

Brian to address your issue right. You're experiencing water splashes out/over transom at speed or prop is over revving on choppy wavy water cond and at close U turns ? Need to determine at which lower leg height is water flow passing by, to check that out.

1-Inflate tubes and keel to its recommended working presure which is 3 to 3.5 PSI with a gauge. Pic 1

2-Trim motor to 90 Deg usually 2-3 holes out transom. Pic 2

3-Sit wife up front towards bow. Pic 3

4-On flat calm water cond go full throttle, while sitting on middle aft deck pull head out and report at which lower leg if flow passing by Pic 4 Check if passing over or under small upper plate or upper AV lower plate.

5-Will know if in need to raise or lower transom accordingly to cancel whichever of both mentioned issues is your current problem.

Once motor/transom height is corrected, will share some tech tips to improve the 5 HP motor performance and get the best of of that combo. Happen to own a 320 Mt Sib and a 5 HP motor...

Happy Boating
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Old 20 June 2019, 19:35   #15
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Brian
I’m on the East Coast and have basically the same setup.
The problem you are having is the same one most of the sailboat ⛵️ people here have.
Our 5hp 4 stroke outboard weighs 49lbs.
A 15hp 2 stroke weighs 35lbs
That is a huge weight and power difference.
A 15hp 4 stroke is 100lbs.
I have found that I need to move as much weight as possible forward.
If running by yourself you will need a tiller extension handle.
Also make sure your propane tank is sitting upright and not on its side
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