Bahri Outboard Speed

[tajrin]

Hi People,

We have a 13ft bahri type boat with two outboards one main motor: Mariner 9.9HP and one spare: Tohatsu 3.5HP.  Both are 2-stroke.

We noticed that when using the 3.5HP at full throttle we roughly get 5 knots.  However when we use the 9.9HP at full throttle we get around 8 Knots.

Is this normal?  Should the 9.9HP give more speed?

[markus]

With the 3.5hp  approx 5 knots is perfect on your 13ft
With the 9hp you should reach approx 12-14 knots

[tajrin]

tnx markus....do you have any suggestions/modifications that I need to verify?

What we notice is that when the 9.9 is at full throttle the front of the boat rises, while the back end goes a few cm in the water.  We have set the motor trim to minimum, but this still happens....could it be that he motor is mounted incorrectly?

[fishfinder]

probably the motor is not positioned to give the best performance. check that the motor fins are approx 1 inch below the bottom. else if this is not enough you will have to attach a slanted wooden board additional to the motor bracket so that the motor could be trimmmed more closer to the boat.

[malvizzu]

Try fitting trim tabs to the 9.9HP so that boat will not rise that much from front. It helped when I get the frejgatina with a 9.9HP.

[skip]

One inch below the keel will cause drag - Not good and I would tend to avoid setting the boat up like that unless this is specific to Bahri's. The main anti-cavitation plate should be at best in line with the bottom of the hull V or slightly above by say one hole as a starting point.

Measure the distance from the left hand side of the engine mount to the outer part of the boat and do the same on the right hand side and compare values. It could be that the main engine is offset too much and so your thrust is off center by too much.

Assuming the engine is working fine and generating the correct power as designed, I am inclined to say that you possibly have the wrong prop on the boat and you are not allowing the engine to reach maximum WOT RPMs. With a 9.9 2 stroke the Bahri should plane easily and do over 10 knots, so you really need to know what RPMs your engine is getting you up to with the current prop and weight loading. You need something like a TinyTach to read the RPMs, perhaps some one has one here http://tinytach.com/gasoline.php as the WOT range for your engine should be around 4500-5300rpm

Also have a look at what prop you currently have on the 9.9. It should be something like a 9.2 diameter x 7.8 Pitch or thereabouts.

[tajrin]

Tnx all for your replies.  I have attached two photos I took before summer so maybe you can tell me if the motor height is mounted correctly.  To me it seems that it is.

Malvizzu: Regarding trim tabs are you referring to something like these:  http://shop.inflatableboatparts.com/product.php?productid=883&cat=0&page=1?  If yes, should I try them out and find it of no use, what do you do with the mounting holes...leave them open or close them with a nut and bolt?

Skip: tnx for your advice....I need to get a rev counter to check.  I am assuming that the engine is mounted at the correct centre as while driving it does not pull to the left or right.  Also keep in mind that the auxiliary motor is mounted at the right of the boat and it still is able to generate 5 knots.  The prop specifications you specified should be engraved on it?

 

[tankard]

I have a bahri with a 2 stroke 25 Johnson. At WOT and just one person on the boat on calm seas is very dangerous cause only the prop is in the water!!.  I wouldn't recommend.
Tajrin: With a 9.9 on a bahri with 2 persons at the back plus aux motor and a tank full of petrol its very difficult to get on a plain.

[tajrin]

tankard: thankyou for your comment, but my problem is not about plaining but the fact that a 3.5hp mounted on a spare motor bracket gives me 5knots, while a 9.9hp mounted properly at the centre gives me 8 knots.

[busumark]

did you try it on your own because if the boat does not plane as it seems it is not doing the way you described it you will not do more than 8 kts. If you tried it on your own and it did not plane the problem is one of the mentioned above

[tankard]

Dear Tajrin. You said that the boat is a bahri type. Does your boat have a planing hull or a displacement hull?. Cause it makes a huge difference. Cause a lot of people describe their 13 foot boat as a bahri. Are you sure the 9.9 is working fine or maybe running on one piston. Has it always been like this or did u just purchase the boat? More pics will surely help.

[malvizzu]

Malvizzu: Regarding trim tabs are you referring to something like these:  http://shop.inflatableboatparts.com/product.php?productid=883&cat=0&page=1?  If yes, should I try them out and find it of no use, what do you do with the mounting holes...leave them open or close them with a nut and bolt?

Yes those are the trims tabs I was referring to. Good question it's a risk you have to take when drilling into the outboard plates but I'm more than sure that it will work, at least on my frejgatina with a very round hull it helped. But leave the trim tabs as a last resort. Maybe the advices of the other members, especially the size of the propellor, should work better. Good luck mate. Hope you'll find a solution.

[tajrin]

tankard: attached is a picture showing my bahri and its hull.  I purchased the boat this summer and its my first experience so excuse my amateurism.

[tankard]

@ Tajrin:  Looks like a cadet not bahri but it doesn't make a difference cause the hull is identical , only the superstructure is slightly different ( wide side benches).
If I am seeing correctly you have an extended transom therefore I am assuming you have a long shaft motor. Most probably you have the anti cavitation plate not in line with the bottom or the motor is not trimmed correctly due to the transom extension angle.
Another reason could be a wrong prop with different dia and or pitch. Maybe the long shaft outboard was as an aux motor for a larger boat and a sail drive prop was fixed.
 

[tajrin]

Hi guys,

A little update on this thread.  I think my worries were all for nothing as the main problem was my inexperience (or first year experience with a boat). 

The actual problem was that after 2 months at sea lots of grass got attached to the hull bottom lines and grooves.  (seems the anti-fouling was not doing a great job).

Another problem was that the motor impeller intake guard filter was half blocked by grass, muscles and dirt.  Once all this was cleaned and another trial was performed the boat managed to get a speed of 14 knots with two persons on board, around 40ltrs fuel, spare motor and fishing accessories.

Thanks all for your help and discussion participation.