Yamaha F9.9F dual thrust prop for ETAP 22?

[Inselaffe]

So, now the next step...

I have bought the 4-stroke Yamaha F9.9F.
http://www.yamaha-motor.co.uk/products/marine/outboards/four_stroke/f9.9_f8.jsp?view=tech_specs

It is not the high-thrust version of the engine, but I will put a Yamaha Dual Thrust Prop on it.
http://www.yamaha-motor.co.uk/acces...ne+Acc/Boat+&+Out/Propellers//PRP-DLTHR-SE-RI
Good picture here (for D.A.R. estimate)
http://www.buitenboord.com/yamaha-o....html?osCsid=f0b6f9778b5ec0cc66067fdd62da665a

I am now sure that the options available for this engine are:
(a) 9x5-N
(b) 9x7-N

The boat is an ETAP 22, LWL 18.4', Displacement 1500kg (book value of 1250 plus 250kg for stuff & people)

I have been messing around with the Surfbaud Excel prop calculator
http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls
and the Castle Marine program
http://www.castlemarine.co.uk/pitch.htm

But am still not sure of the best solution.

I think that if I get the 9x7-N then I will reach hull speed at lower revs, which may be more comfortable?

Any comments, especially what I will gain/lose with one or the other?

Thanks

 

[VicS]

The highthrust FT9.9 not only has a dual thrust prop but has a very different ratio gear box. 13:38 compared with 13:27 for the F9.9.

This means that for the same engine revs the propshaft of the FT version will be turning only at about 0.7 of the revs of F version

Perhaps you should also be changing the gear box to the one for the high thrust model.

What size ( diam and pitch) props are fitted to the two models as standard.

Maybe they are the same or similar and the high thrust characteristics are achieved solely via the different gear ratios in which case you will need a prop with a pitch only about 0.7 of that of the prop normally fitted to the F9.9.
The slower turning props for the FT may be a different ( larger) diameter. Or larger blade area. It may not be right to fit a larger diameter prop to the F9.9 ( it may not even be possible)

Are there are any other differences in design between the F ad FT versions Any subtle differences in carburation or ignition for example.

The dual thrust feature simply gives you as much thrust in reverse as you have in forward.

 

[Inselaffe]

I am sure it is possible, and advantageous to use the Dual Thrust props on the F9.9F; Yamaha themselves say so and I successfully used one on my F9.9A for years.

I will be sticking with the normal gear ratio Long shaft engine, as I got a good deal and it is equivalent to the one I had before which worked fine.

The two Dual Thrust props that will fit the F9.9F (as opposed to the F9.9A, F9.9B or F9.9C or any of the High thrust versions) available are (from the Yamaha parts exploded diagram, which shows the F9.9F with a Dual thrust prop):

(a) 9" diameter x 5" Pitch (Yamaha type N prop)
(a) 9" diameter x 7" Pitch (Yamaha type N prop)

Also here (although F9.9F is not mentioned it is the same as the F8C as this is the engine that these newer, lighter 9.9's are based on):

http://accessories.yamaha-motor-europe.com/files/documents/EU/2009_Prop_DualThrust_090618.pdf

i.e I have a choice of 5" or 7" pitch, but the diameter is fixed at 9"

Which one?

The 5" pitch will require more revs for hull speed than the 7", as I see it.

What are the possible disadvantages of taking the 7" pitch one so that I get hull speed at a lower revs (and less strain on motor ears and vibrations)?

The calculators both suggested near to 9" diameter and 5 or 7" pitch depending on the max revs I input.

So really my question is, 'What are the possible dis/advantages of each or the two choices, 5" or 7" pitch?"