9'6 rib honda 5hp 4 stroke more speed please

[DonNZ]

Hi folks

I have inherited a 9'6" rib ( glass over ply ) tender in excellent condition powered by excellent '97 honda 5hp 4 stroke. Currently it goes 10 to 11 kph / 5 kts (gps). I have not confirmed the engine rpm as waiting on a tacho. std prop ( tidy condition)

I would like to go faster if able and have read the ideal hieght of the antiventilation plate (AVP) is top just dry at wot. The AVP is in line with the bottom of the keel. At wot the AVP is prob 6 " below the water line.
I'm guessing this measure of AVP dry may be when the boat is on plane, Not sure if I can achieve this unless I find a patch of downhill water.

My question if I was to raise the motor so the AVP is in this ideal position then the top 1/3 to 1/2 of prop will be behind the transom and not in laminar flow water. Do I have to go without this ideal postion due to lack of HP ? or would it be worth my while to lift the motor say 1" and try it ?

Thanks

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Welcome,
I think a very good investment would be tiller extension.To get your weight forword.
Your motor on an ideal boat could do about 12 mph planed out.We had a steel boat with a old 5 gale with an extension it would pop on plane with a 180 pounder in the middle seat.
Your rib is not ideal other than the light weight.At about 5 knots, 5.2 mph your not planed.
Be sure the boat is at max pressure.Play with the trim for best speed.As far as the height see if it will plane then we'll see about the height.

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Thankyou Spike

I've tried moving my weight ~190lb plus my daughter ~70lb well fwd as by setting the tiller straight we steer quite well with leaning. I've had my prop to the local prop guru and that gave us another 1 kph top speed with much improved accelleration from a sharp turn. I'll retest speed 1 up next outing..

Boat total dry weight around 180lb (glass flat floor) including 27kg engine, fuel is as far fwd as possible. Wish boat was 50 ~100 lb lighter on slip !!
I've been watching youtube vids of the Tohatsu 9.8 hp 2 stroke at 26 kg US$2250 here !
In following seas we get overtaken by the wave. Another month and we'll be into summer and I'd like to try and pull my girl in a tube.

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

when you get your rpm figured out we may be able to figure a prop change.Do you know your present prop we'll need that to figure how your setup works.

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

True, I do understand I'm blowing in the wind until I advise wot rpm, water speed and prop size ...
Next outing, maybe tomorrow.
Props to fit honda 5hp are like rocking horse manure here. Genuine honda prop here US$200.

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Ok We'll be waiting. With any luck maybe you can get the prop reworked.
Hey How about that Gin Wigmore.
I can't get enough of her songs,looks and arangements.

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Results from todays run.
Shifting seating position to find maximum engine speed leaning to lying down to reduce windage.

prop 7 7/8 x 7 1/2
wot engine speed 5400 rpm
Gear Ratio: 2.11:1
max ground speed 14kmh
boat weight dry 180lb
9'6" rib glass over ply
http://www.outboard-engine.com/enginespecs.php?recordID=89
above spec for max rpm 4500 - 5500 rpm

Sorry to burst your bubble but you'll need to take a number
Also Gin's a Sydney girl now. Got plenty of spunk too !

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

OK by the prop calculator I use you have about 50% slip ,a rediculas number.
Typical number would be about 6 -12% at 12 % 5400 rpm would be about 15 mph.
Assuming your numbers are right. The the slip numbers seem to indicate something is disturbimg the water around the prop.Is it possible the bottom is distorted either from poor
condition from poor storage or a weakness of design or inflation pressure?
5400 is singing pretty good does it sound like its making that kind of rpm?
Are you sure the tach is set right?
Are you sure the prop is 7.5" pitch?
Is the motor trimmed in all the way (prop closest to the boat)to help it plane?
You could try raising the motor but its hard to believe you could almost double your speed that way.Puzzleing. Something doesn't add up.

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

No I'm not sure about the tacho. I was using a backup that is unverified.
I was dissapointed when I read the rpm as I figured there was nothing left ! I hope it is incorrect.
I'll try to obtain another instrument to verify.
The prop dimensions are from whats moulded onto one of the prop blades.
on a different blade is written 200 x 190.
trim position is full fwd / prop as close as possible to the boat.
The hull is fiberglass over ply and ok condition, it has a few scratches but not so bad.
Pontoon inflation was taught enuf, I've no gauge but by feel its ok with my 1 month experience

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

200 X 190 is milimeters.Assuming 12% slip for a 7.5" prop to make 14km/8.7mph the motor would only be turning about 2900 rpm.
Are you sure its running on both cylinders? What is the idle rpm in gear?

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Thanks Spike

I was keen to know the math result to verify.
Good, something must be wrong. Might go faster yet !!
My Honda 5hp 4 stroke is single cylinder, I'd have another
The Tohatsu 9.8hp 2 stroke 26kg, 1 kg lighter is winking at me.

What I used as a tacho counted 10800 so I assumed 1 spark per revoloution.
It is via a inductive pickup on the spark plug lead so may be false triggering.
I'll improve things.
Good thinking to check the idle.
I'll try from the supply to the low oil pressure led as I think it's via the coil low tension so may be less noise.

I wondered .. . as I have recorded a short video, the engine sound / note will be a frequency,
Any idea what the web address for the play your sound and it tells you the frequency ?
Multiply that by 60 it'll be rpm ?
I have a uke tuner, hmm

Another awesome sunny day I get to play, nice.

Thanks for your ideas.

Regards Don

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Just had a thought
Gear Ratio: 2.11:1
Is your 2900 rpm calcultion considering the gear ratio ?

Didn't make it onto the water.

Regards Don

 

[steelespike]

Re: 9'6 rib honda 5hp 4 stroke more speed please

A 4 stroke is one spark every other revolution.
Your 10800 is either set wrong or picking up some stray pulses.
A 4 stroke on a 2 stroke setting would be half the actual rpm.
I think I used 2.15 but rpm is esentially the same.
On further thought An rib with its tendency to sometimes struggle to plane
and not usually the most efficient hull and yours not the lightest for its size maybe
14km/8.7 mph is about it.thats allmost 9 mph. 12 mph being typical for a 5 in an average setup.You could try to raise the motor 1/4 to 1/2"(6 to 12 mm ?) at a time to a point just before venting becomes a problem.Just add sticks to the top of the transom to hold the height till you tighten the clamps.Be sure they have a good grip.If the motor is really low you might pick up a little more speed if it gets it onto a real plane you might pick up more.
The Tohatsu 9.8 is probably a better choice if the boat is rated for it.
It will use twice as much fuel but may get there twice as fast.Of course you could
cruise at partial throttle and the motor would be loafing and saving gas.

 

[DonNZ]

Re: 9'6 rib honda 5hp 4 stroke more speed please

Thanks Spike.

I agree a 4 stroke is one ignition spark every 2nd revoloution.
Simple ignition 4 strokes (triggered from crank at 2 revs per ignition sequence and not via the camshaft at half the speed ) I've serviced have been what's called waste spark ignition. Meaning one spark is wasted as it's produced on the exhaust stroke. Otherwise the simple 4 stroke ignition sytem would need to be clever enough to know when your on the compression stroke to turn on the spark and turn off the spark on the exhaust stroke.
The Tohatsu or similar is quite likely, I'll have to wait for next winter.
The extra fuel consumption is not a problem. I have the 3 gallon / 12 litre tank and it's lasting 3 weekends 2 - 4 hours motoring both days ) so currently alot larger than needed.
Re the rating, no labels at all on boat, looks less tha n 10 years old and been stored away from the sun. Solid well built transom with good gussets to a solid flloor, so strength wise it'll take a 15 but my back wont. Most important the extra 30 ~45 lb hanging over the back will equal less freeboard with engine off. Not sure my baling speed, endurance and lunch would be ready for it !
Hopefully I can sneak out today for a play.

I can access a OK sailing dingy I could put the engine on thats probably 1/4 of the weight to assess speed / weight change. I'll confirm rpm first.

Regards Don