outboards

Outboards or inboards, electric, fuel or diesel, cavitaion and torque talk
44c
Posts: 1148
Joined: Tue Aug 08, 2006 5:08 am
Location: Hervey Bay, Qld

Re: outboards

Post by 44c » Fri Sep 09, 2016 8:23 am

isladelobos wrote:Hi.
Im from Spain.

This is my first post here.
Very informative thread.

What about the new 9.9 Yamaha High Thrust?. I see some ones installed on Seawind but not listen good or bad reports for now.

I understand with this new 9.9, the engine need more rpm's for the same effort?

Free beers today.
Hi, welcome to the forum. Can't speak from personal experience, but some people I know have used both the new and the old Yamaha 9.9 high thrusts, and are happy with the new one. I don't think the RPM's are much different.

They do report very good fuel economy with the new motors.

isladelobos
Posts: 3
Joined: Fri May 06, 2016 7:45 pm

Re: outboards

Post by isladelobos » Tue Dec 06, 2016 9:06 pm

Some months ago i send a mail to a Parsun dealer for question about if their have a 9.9 (323cc) high thrust model on his catalog.
And his response was: We don't have these high thrust engines, can I know how many pcs you need? If large quantity, we will talk to our general manager about this.
And my response was: Sorry, im not a dealer.
And his response was: We don't have other customers request this high thrust model recently. We will let you know if we have plan to produce this model.

I don't know if this response can be interesting for some catamaran makers but if i see in the shop i gone to take two for tests.

44c
Posts: 1148
Joined: Tue Aug 08, 2006 5:08 am
Location: Hervey Bay, Qld

Re: outboards

Post by 44c » Wed Dec 07, 2016 7:47 am

After having read through this thread a bit, I thought I'd add some real-life experience.

After 6 1/2 years full-time liveaboard cruising, our boat now has 31,000 miles on it, the engines are around 1500 hours.

Despite Sandy D's protestations, the Honda 20's with Honda's "Powerthrust" (Slightly larger diameter and lower pitch) props do an excellent job. The prop's are available in two pitches. 6 3/4" or 7 5/8". For our boat the 7 5/8" work well.

We get a shade under 8 knots at around 5200 RPM on one motor, nearly 9.5 knots at 5500 on both, flat out.

For economical motoring in flat calm, around 5.5 - 6 knots at 4000 RPM on one motor.

Fuel economy is reasonably good, we seem to average around 2 litres per hour per motor.

I firmly believe leg length (and bracket size) is crucial. It doesn't matter what gear ratio or size prop you have, if the prop is out of the water! We've motored into 35 knots wind with decent waves, and only experience minor, momentary prop aeration. And had plenty of thrust for the job, on two motors. One motor would have struggled.

Some OP's experiences. Brett on "Swanning Around"" switched from the Honda 20's to Yamaha High thrust 9.9's, due to poorly made leg extensions. (Not made by Honda). He reports that there is definitely less "grunt" available from the 9.9's. (As you'd expect, they're 1/2 the horsepower) He's very happy with the fuel economy though, and they still do an adequate job.

Bryan and I fitted Tohatsu 20 4 strokes on "School's Out". They have a genuine Tohatsu 25 inch leg extension, and Solas high-thrust 4 blade props. (Very similar to Honda's powerthrust props.)

I like these motors. Performance is pretty much the same as the Honda 20's, (specs are near identical) but they're easier to install, with a much more compact bracket in the power tilt version. Th emore compact bracket means the effective leg length ends up being pretty similar to the Honda, with the 28" leg. I like their control cable mounting system better, (Honda's is pretty crude) and servicing is easier, oil filters can be replaced without having to remove the engine sidecover. They do seem to be a bit noisier than the Hondas.

Hope this helps anyone looking to buy motors!

Bingo
Posts: 15
Joined: Tue Dec 06, 2016 8:26 am

Re: outboards

Post by Bingo » Fri Dec 09, 2016 1:58 pm

Gday al . Encouraging performance figures. Currently doing hull extensions on one of bobs sailing boats (white knuckles).Was 8.8 metres now 10. Chose not to go with full blown powercat hulls instead modify existing shape.350 ml draft,150 ml max under water at the stern 400 ml wide hulls. Reasonably confident with my expectations of achieving similar figures. 8 knots 4500 rpm one engine 12 knots 4 to 4500 rpm two motors 1.5 to 2 mile per litre. I to raced (off-road) motorcycles for many years and was a huge Yamaha fan, had larger Yamaha horsepower motors on several boats in my 30 years fishing the territory but agree totally on the 20 Honda being the no brainer in the smaller horsepower class(leg length weight etc) and will no dought get an upgrade (fuel injection) soon in line with other manufacturers . I know your longer (important) but I reckon I'll weigh half as much with slimmer hulls. It's 400 mile from here (Darwin) to Napier broome bay (refuel) in the Kimberley allowing for side trips up the Berkeley and king George rivers 250 to 300 litres max can carry 500 hunji. Oh and by the way forgot to mention we briefly met at I think it was montys had coffee in the cockpit of schools out whilst you were doin repair job I was with my brother in law Craig. Happy days, Steve

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: outboards

Post by mahnamahna » Fri Dec 16, 2016 12:32 pm

I have a question I cant seem to get a straight answer on, perhaps it really is because there is no straight answer. I asked 6 different people so far, some dealers, and get 7 different responses. The one unanimous reply though is you have to try each option for yourself and see which works best for you. Which is fine but there must be some scientific formula that would give me the theoretical answer. So here goes.

I have 2 2007 (older larger head model) 9.9 high thrust Yamaha outboards. One came with a prop. Its a 11 1/4 inch diameter by 9 1/4 inch pitch. The letter at the end is R which I am told is for a 25-30hp and the designation for 9.9-15 is J.
I have spoken to a local Yamaha dealer and he said the 9.9 high thrust does take the prop I have and another is $150 and he could order me one.
I can also get a second hand one on ebay for $120.
However I can also buy on ebay 2 new props for 9.9 8 spline motors that are 9 1/2 in diameter by 8 inch pitch for only $60 each.

My understanding is that for our applications, that is pushing a 40ft cat at relatively slow speeds that dont plane, we dont need top end speed we need down low grunt and the lower the pitch size the more torque the higher the more top end speed (think tinnie on the plane that a 9.9 might also be asked to do).

My question is which is better for me, bigger prop diameter but also larger pitch or smaller prop diameter but also smaller pitch?

I also figure the bulk of the time I want these props to work at low speed but in a marina space flicking between forward and reverse as I use the motors to steer, however I dont want to be trying to push into a wind and current and find the props just wont push the boat fast enough and I am not making any ground.

Any ideas? In short which variable is more important diameter or pitch.

(my solution is going to be I am going to buy one of the big diameter props to match the one I have and a pair of the smaller diameter as spares and try one day I may experiment, but I will have my spares albeit in the different sizes. I am just curious as to the theory).

44c
Posts: 1148
Joined: Tue Aug 08, 2006 5:08 am
Location: Hervey Bay, Qld

Re: outboards

Post by 44c » Sat Dec 17, 2016 7:56 am

For our application, we need bigger diameter, smaller pitch, compared to using the same motor on a small planing craft.

The diameter of the prop is limited by the space from the prop shaft to the cavitation plate. You simply get the biggest diameter prop that will fit.

Then it's just a matter of matching pitch to the gear ratio and projected boat speed.

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: outboards

Post by mahnamahna » Sat Dec 17, 2016 9:28 am

Sorry Allan just to clarify, largest diameter that fits albeit if the pitch is also slightly larger is preferable over smaller pitch albeit also with smaller diameter? Just trying to ascertain which variable diameter or pitch has the greater influence on our situation.

Finally
Posts: 223
Joined: Sat Jul 04, 2009 9:24 am
Location: Cairns

Re: outboards

Post by Finally » Sat Dec 17, 2016 1:09 pm

I'm no expert but thinking this through I would have thought for thrust the more surface area pushing water, the better. So diameter would be more citical. Then you match the pitch to the diameter / surface area of the blades to suit the motors at working RPM's.

There are a number of prop calculators on the web: https://www.vicprop.com/displacement_size.php as an example.

David

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: outboards

Post by mahnamahna » Sat Dec 17, 2016 2:27 pm

David my dilemma is that in a budget prop it seems I have to choose which parameter I prefer. I can have the max diameter of 11 3/4 with a pitch of 9 1/4 or even cheaper is a smaller diameter of 9 1/2 with better (for torque) pitch of 7. Price goes up dramatically if I want both larger diameter and smaller pitch so was just curious as to which had the most effect. Seems its diameter. Thanks all.

mahnamahna
Posts: 580
Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

Re: outboards

Post by mahnamahna » Sat Dec 17, 2016 2:39 pm

Wow David no wonder nobody really knows off the cuff. Just looked at your calculator link. Surprised they dont need the skippers jock size, they ask just about everything else.

Post Reply