Question regarding Pitch and 3 versus 4 blades

[PartyBarge]

Good late afternoon all. Sorry this report back took so long but we finally mounted the 3 blade 15 pitch Rubex 3 late last Thursday afternoon with very light Lake traffic and zero wind. I was onboard solo with my 25 gallon fuel tank at 3/4 full. WOT brought TACH to 5600 with a Top Speed of 26MPH. As for the 17 pitch prop, knowing that mounting it will almost certainly drop the WOT RPM back down to somewhere between 5600 and 5000 (that the old chewed up 19 produced) I'm thinking why even bother. I'm happy with the #'s the 15 has delivered. Factor into that the 15 was brand new in the box & all I paid for it was a bottle of decent 12 year old Scotch. That's it, just wanted to report back with the #'s. Thanks again to everyone for their feedback and have an excellent evening!

That's really good news! Even at a low weight, 26 MPH is very good and no better proof that 25" tubes with lifting strakes work. As noted above, additional load, and it's distribution, will reduce top speed. With only 90 HP on tap, expect to loose 4-5 MPH with, say, 6 adults + beer (scotch?). Hard to say how additional load will effect RPM with that prop. It's possible that slip will increase enough that WOT RPM loss will not be a big deal. I'd love to see you try our Spitfire 13p pontoon prop (4-blade aluminum), but it's going to be in use for the foreseeable future.....

I expect our next pontoon will have larger logs (currently have 23") and strakes.

 

[Greenchileman]

Thanks for the feedback. This gets me to thinking (too much according to my better half) so I located a 2004 Bennington catalog & found the specs for our boat. We have strakes on both sides of both toons & because of this, according to the catalog, we should be running w/at least 115HP (the "safety" placard states 130 is the max for this particular craft) & being fairly new to boating I guess that would be to fully realize what the strakes offer. That said, we currently have 450 hours on the 90 and it (knock on wood) receives annual maintenance & runs like a top so unless it blows up (which I hope doesn't happen) we aren't willing to drop the $ "mid-term" to upgrade to a 115. In fact, before we'd purchase another motor for our '04, we'd be shopping for a later model Bennington (my wife is close to being onboard with this)! Last but not least, I'll look into the 4 blade "Spitfire" Prop & thanks again for everyone's feedback!

 

[PartyBarge]

As you look into some prop tinkering, be sure you are looking at the pontoon version of the Spitfire/Nemesis. The Spitfire line is so popular that there are several somewhat different versions/focus. Also for modest power, intended use really matters because we can't just blow away a mismatch with HP. As an example of that, for our normal use the 13p Nemesis is excellent for top end and best econ at idle or a comfortable cruise (suspect that ~15 MPH would be about the same for you too). The decision for a 13p was not easy as I had to anticipate the Nemesis slippage would be way less than the basic 15p 3-blade we were running then. All that worked out very well.

Knowing what I know now, IF we normally ran with a moderate to heavy load and/or towing, I would go with a 12p Nemesis. Light-weight top end would be no different and light-weight midrange fuel economy might suffer a little bit, but the 90 Yamahas are so good on fuel that is of little concern. It's only speculation but if you do try a pontoon 4-blade the 13p or 12p decision will likely hinge on most common use. Expect a Nemesis plus hardware to cost you two+ bottles of 12-year....(Don't know what state you are in, but 15-year Glenfiddich should be within reach...you won't believe how good that stuff is!) Maybe the first order of business is to decide on the prop or the Scotch?

 

[Greenchileman]

First off, thanks for the tip re the 15 year old Glenfiddich, haven't tried it but will go across the state line into SC (lower sin taxes ) to pick a bottle up soon. Also, you got me thinking again. About 7/8 of the time it's 2 onboard (my better half & yours truly) going out for a cruise in the late afternoon (no towing going on) followed by drifting and watching Sunsets. The other 1/8 of the time we have guests visiting and then it will range between 4 and 8 total onboard doing pretty much the same float plan as outlined above. Based on this, it sounds like the 15 pitch is AOK for now. That said, I'll pick up one of the 13 pitch Spitfire toon props to at least have available when I want to "tinker". Thanks again for the info!

Spitfire Pontoon (13.8 x 11") MERCURY RH Propeller 8M8026560

www.boatpropellerwarehouse.com

 

[PartyBarge]

Ahhhh....why settle for just one, a prop and the scotch, well played! The source you linked only shows the 12p in the larger hub diameter (4 3/4" intended for the CT Command Thrust motors, interesting). I thought there was a 12p in the 4 1/4" hub too...guess not. Do a little shopping for a Nemesis, pretty sure there is a better price available including the shaft adapter...although, prices have gotten a little out of control lately....

I hesitate to say this because I don't have all the performance data needed, but you MAY get another 2-300 RPM even with the reduced slip, but likely about the same top end...of course I may have to claim I was hacked and never wrote that....

BTW: we are on Lake Keowee in Seneca, SC.

 

[Greenchileman]

Hello from Lavonia GA! I haven’t been to Keowee yet but have seen photos & it looks Gorgeous. You don’t happen to know a fellow by the name of Roch do you?

 

[PartyBarge]

Nope. Yeah, Keowee is a fine lake, at a little more than 18,000 acers, it's about a third the size of Harwell, but plenty big enough for us.

 

[Dhende3665]

I have read all the posts about prop pitch. I've got a a 2015 Bennington 20sfx, 25" tubes, with a 90 Yamaha. It has a Solas SS 3 blade13.5x15 prop. With 2 on board at wot trimmed out all I can get is 5200 rpm at 24-25 mph. I want to be able to turn at least 6k with 2 on board so when I have 4-5 on board I can turn more than 4900-5000 rpm, which is what it turns now (20-22mph) What prop do I need to attain my goals?

 

[Vikingstaff]

I have read all the posts about prop pitch. I've got a a 2015 Bennington 20sfx, 25" tubes, with a 90 Yamaha. It has a Solas SS 3 blade13.5x15 prop. With 2 on board at wot trimmed out all I can get is 5200 rpm at 24-25 mph. I want to be able to turn at least 6k with 2 on board so when I have 4-5 on board I can turn more than 4900-5000 rpm, which is what it turns now (20-22mph) What prop do I need to attain my goals?

I am not too familiar with that HP range on a dual toon. However, I will say that the problem is having the SS prop that large in pitch with that 90 HP, dual toon set up. Just too much prop to turn.

Switch to an aluminum prop with the lower HP motor. Additionally, 15p seems WAY too large for a 90 HP. Again, just too difficult for that motor to turn, let alone a SS 15p prop. Making those changes will be easier to turn for a 90 HP motor. And at that HP, the aluminum is not going to flex so you wont lose performance like you would if it were on a higher HP motor.

I run a 16p SS prop on my 200HP motor. With a change 3 years ago, I could switch down to a 15p version of that prop to really dial back in. However, I turn 5800 RPM at WOT on a light load with my current 16p. With a 15p, I’d probably bump up to 5950-6000 myself.

Hopefully others with a similar boat, motor and hull set up can chime in with personal experience and give a very refined recommendation. I just thought I’d chime on on what I see as the two main problematic variables for you.

With the right basic style of prop (proper pitch aluminum prop), you will at least get the RPM situation rectified, and then be able to see what that translates to for performance.

 

[Bill N]

A 13" pitch should yeild your desired results without over-reving

Specifically I would try part # 6E5-45949-00-00

 

[Potomacbassin’]

I have read all the posts about prop pitch. I've got a a 2015 Bennington 20sfx, 25" tubes, with a 90 Yamaha. It has a Solas SS 3 blade13.5x15 prop. With 2 on board at wot trimmed out all I can get is 5200 rpm at 24-25 mph. I want to be able to turn at least 6k with 2 on board so when I have 4-5 on board I can turn more than 4900-5000 rpm, which is what it turns now (20-22mph) What prop do I need to attain my goals?

There are tradeoffs with every prop - if your goal is top speed, there are props that will give you that but you may lose some hole shot or fuel efficiency. The same goes for any other goal (many aim for best "all-around").

Generally speaking though you've got a great data point to work from - 5200 RPM indicates too much pitch, not to mention what Viking stated you've got more rotational mass to spin with that stainless prop and could benefit from going to aluminum. Yamaha and Mercury both have prop selectors online that can give you some recommendations, or reach out to one of several prop shops (Prop Gods is one that many of us have used and highly endorse) who have years of experience and can get you dialed in.

Bill N's recommendation is a good one - go down to 13p aluminum and you'll gain 500-700 RPM. Or consider the Yamaha Talon 6EK-45945-00-00 which is a little less pitch and diameter, but is a newer design with the SDS hub (soft and low-clunk on gear shifts) that has more cup and should give you more grip and less slip.