So, a question here.

Lets take a prop, and we'll use 9x6 for example. At 4,000 RPM, with next to no slip, a prop calculator gives a boat speed of 22.5MPH. From experience (if you consider actual slippage) this is pretty accurate on my boat.

Change the prop to a 9x7, and keep all other factors the same. Same calculator gives a boat speed of 26.3MPH.

So in theory (with no slippage), if you turn the two props the same RPM - the higher pitch will give you a greater speed.

If you put each of these props on a direct drive, 1:1 ratio motor, the higher pitch should yield a higher speed. UNTIL prop slip comes into play.

What creates prop slip? If I can turn each prop 4,000 rpms, my first thought is the boat would reach higher speed with the higher prop size. Does its ability (or lack of) to push the loaded hull create additional prop slip - potentially preventing the larger pitch from reaching higher speed?

Even though the larger prop has a high speed potential, could you compare it to tires slipping in the mud and not getting traction? Is slippage the prop not getting a good "bite" on the water to push the boat forwards?

Load creates prop slip. Friction of the boat on the water. Friction of the boat pushing water. And the water density compared to the turning of the prop itself. Prop anglr has a little to do with it, that straighter the prop in the water. The more efficient the thrust is. Denser water = less prop slip. Less hull friction = less prop slip. Straighter prop shaft angle in comparison to the water line = more efficient thrust

It's simple. get a mud motor. shoot ducks in the face.

All factors the same, a saltwater river should give you better bite and speed than a freshwater? Probably an amount too small to notice?

If you have the same boat and same load, will your slip be the same regardless of the prop pitch you are using? For instance, will the slip of boat X and prop A be the same as boat X with prop B?

These questions arise because I ordered new props yesterday to do some R&D, and I want to get behind the principle of how they SHOULD perform before I know how they ACTUALLY perform.

Lets add my personal example to this. I'm running a 9x6 on a Backwater LT with VersaMount, turning 4,000 rpms and 21MPH. I ordered a 9x7 and a 9x8. I think it can push the 9x7, but the 9x8 will likely be too much. I have some mods going on, but more on that later.

Disregard slip, and they 9x7 should be faster at the same 4,000 RPMS. I'll find out how much slip comes into play on this example next week.

[quote="reharbert"]All factors the same, a saltwater river should give you better bite and speed than a freshwater? Probably an amount too small to notice?
Answer:

Maybe fractions of an mph. Probably too small to notice. On the high side of things probably 1-2mph at the most under perfect conditions.

If you have the same boat and same load, will your slip be the same regardless of the prop pitch you are using? For instance, will the slip of boat X and prop A be the same as boat X with prop B?............


Short answer. The prop slip will be different. Depending on pitch, cup, prop size, props with less pitch slip more. Props with more cup slip less


These questions arise because I ordered new props yesterday to do some R&D, and I want to get behind the principle of how they SHOULD perform before I know how they ACTUALLY perform.

Lets add my personal example to this. I'm running a 9x6 on a Backwater LT with VersaMount, turning 4,000 rpms and 21MPH. I ordered a 9x7 and a 9x8. I think it can push the 9x7, but the 9x8 will likely be too much. I have some mods going on, but more on that later.

Disregard slip, and they 9x7 should be faster at the same 4,000 RPMS. I'll find out how much slip comes into play on this example next week.

Answer: disregarding slip.....use the 9x7 should be faster. Factoring in load on the engine, if your engine can even get to 4000 rpm with a bigger prop, it will be faster. If the bigger prop loads your engine too much it will be slower.



It's simple. get a mud motor. shoot ducks in the face.

This motor will actually get to 4400, but I don't want to push it yet. That will come soon. Thanks for the discussion. You've help clarify some things I've thought. Either we're on the right track, or have no idea what we are talking about.

I'll let you know how it turns out next week. Maybe I can get me a little video comparison.

You can turn that 9 x8 I'm running a modded 23 ( stout as a mfer ) turning a 9x8 all day long , I'm actually going to try backwaters new 10x6 on my motor for them. The 9x8 is for stock 35 hp , the 10x6 is for stock. 37 hp.
10" on left, 9" on right
H Finn

Usually around. 26mph cruising speed empty .

H Finn

I talked to Jake yesterday for a while. I love calling those guys, always super helpful. He was fairly confident the 9x7 would be great, but expressed concern of the 9x8 bringing it to its knees - as he put it. He also said I could order both, and run them. Send back either or both if I didn't like it, as long as they weren't beat up. Would refund full price, minus shipping. Pretty good deal and customer service in my book.

Finn, do you get as many surprised people as I do with your Backwater?

The speed always impresses, as well as the shortened length and compact design compared to most long tails. It sure has been a head turner.

The predator grabs a lot of attention too in my case.

It was a stock. 23 hp vanguard swomp ss frame . I modded after. 50 hours .

H Finn

I'm running a 670 predator. I'm actually getting ready to change the valve train, custom exhaust, and bolt up a mikuni.

Let me know how it goes. Later on I am gonna send you some pm's with a bunch of questions. I'd like to learn more about the options s these things have available and what I can do to mine

It's simple. get a mud motor. shoot ducks in the face.

*Kind of got side tracked from the original topic.


I'll make a thread for 670 pretty soon once I have everything laid out for some photos; and ready to bolt it all on. There really isn't much out there on the 670, and not many people have messed with them. I've looked all over the place. Vegas Carts did limited testing on one, but deemed it wasn't worth the money it would cost to mod one. They blew two of them once they hit 5k RPMS. I want to try to keep mine at 4400, maybe 4500. I also found a guy who has been running one with no governor in a golf cart for about a year now with no problems. I know several guys who have been running them on Swamp Runner kits since a few months after their release, with no problems.

I do know the GX390 springs and pushrods fit, so that's a good start. The carb and exhaust should yield good results, I'll let you know when I get it finished up. I'll take a look at the heads when I pull them off, and I'll probably deck them while I have it apart.

I ran into similar findings with my research on upgrade...... not much

It's simple. get a mud motor. shoot ducks in the face.

I can tell you I pulled all the metal out from under the air filter, tweaked the governor, and added an electric fuel pump. I've been running 4000 to 4100 WOT for 30 minutes at a time easy, and have about 30 hours on it now no problem.

I had a 20hp Honda (620) Longtail prior to this surface drive setup and I can say without a doubt that that motor in bone stock form would handle 6,000rpm. It floated valves at around 5500 but never had any ill effects. I probably hit 7K a handful of times after hitting things and knocking the prop out of the water. I had a GD/BW style throttle so I couldn't just let it go and slow the engine down. The governor was also completely removed from the engine. After some free mods and carb tuning I turned a worked over 9x4 prop in the 4200-4400 range unloaded and ran right around 20mph on a hooked 1548 Weldbilt hull. It would turn 3800-4000 loaded up with 2 and gear and run 17-18. I could have probably turned more prop and gone faster but I would have given up a good amount of load carrying ability I feel

I don't know how similarly they are built inside or how well they are balanced but if the Predators are similar to the Hondas in that regard I'd swing it to 5K and wouldn't bat an eye.

Well I pulled off the factory supplied 9x6 this weekend. I was getting 21 steady at about 4,100 with this. Take off was fine. Really, no real complaints. Just wanted to try a different prop.

Put on the new 9x7. My tach got ripped off, so I don't know the RPMs I was turning it. I did bump my top speed to 23, with an occasional bump of 24. Bottom end didn't seem to be affected at all, and motor didn't seem to be bogged down any either.

Should have the 9x8 today and I'll test it tonight - hopefully. After that, another major change will be coming.

I got the 9x8 in, but I ended up swapping the Predator with a Subaru before I tried it.

It's only temporary, and I'll continue with the Predator R&D shortly.

Try it out. , that 28 might swing it good, you coming to setx Saturday?

H finn

I will, assuming I finish getting this thing put together. I ran out there Sunday afternoon. Water was high and choppy, and the wind was kicking.

Ran it Saturday. , just for a little while , still dialing it in . But gains were noticed. Will have more to post after Setx this weekend.

H Finn

Thanks Huck. Have fun at the ride.


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