Prop question

[Flex]

Currently I run a 4.55m seajay with a 50hp yamaha. I've had this boat for 12 months and I've started to use it ona few fresh water lakes and im noticing its top end speeds seems a tad lacking. Im rather happy with the setup, but it feels like it should get a bit more.
Hull is recommended for 40hp with a max of 50hp. It only weighs 275kg+myself,fuel and a bit of gear. So it'd be lucky to weigh 450-500kg.

At half trottle I push 40km/h which is pretty damn good.The boat gets on the plane basically instantly. When I opened the throttle up to full from half I expected to top 60km/h, but I hit a max of 50km/h.

It feels like the hull is almost being Held back when I run at full throttle.

I dont expect to run at 80-100km hour, but on a deadflat surface 50km/h feels really slow, considering I go 40km/h at half throttle.

I run the standard prop that came with the out board a 10/3/8- 13.

any suggestions on what size to run to get a tad more top end?

[FNQCairns]

Flex you will need to know what RPM the engine is doing at each speed.

That said you can guess although it may end up a costly exercise if the resulting change results in an engine that luggs. It could be lugging ATM.

cheers fnq

[Flex]

as its only a stock 2 stroke I have no way of determining how hard its revving? How do dealers work out what props to use?

[FNQCairns]

Yeah it's hard without a tacho, you can fit one though, in theory only if your engine is doing 5500rpm with 10% slip at the13p prop you should see 53kph.

To go further is not really possible without an accurate RPM reading, you could on the surface of what I wrote above raise the engine a little it may find you (depending where you are atm) a couple of extra km/h in efficiency higher up in the rpm range.

Only a 15p will see 60kph from that engine.

cheers fnq

[Flex]

Thanks FNQ. So looks like im pretty much stuck with what I have.

Just seems weird that the top half of my throttle only gives me such a small gain in speed, but my fuel consumption goes up significantly if i run at full compaired to half/3/4.

not a major issue, 50kmh is still damn good and alot of time I cant go that fast due to conditions

[Wahoo]

Hi Flex, you will feel the difference on proformance from salt water and fresh water, Scott is right on the money with the props, should change his name to "Prop God " ...........LOL

Daz

[Jabba_]

Daz, how does freshwater effect the performance????

[FNQCairns]

Yeah good thought Daz, he may see 53km/h in the salt.

BTW you embarrass me..LOL, hows that plastic 2 blade working for you.

cheers fnq

[Flex]

On another question. Will different gearing, engine capacity affect speed a great deal? EG 50hp Suzuki is 2.27:1 with 814cc v say a Yamaha at 1.85:1 at 996cc with an extra cylinder . So does the ratio's mean 2.27 turns of the prop per turns of the crank? whats considered the better ratio and would cc's have an impact on performance? if they are both rated at 50hp?

[Rob128]

Hey Flex,

I have also noticed a difference in the way the boat / motor handles Salt to Fresh water. I go out in the bay often and also upto Somerset Dam. Two factors I have learned and appreciate which play a major role;

i) Bouancy of fresh water Vs salt water = Boat sits lower in fresh

ii) Atmospheric pressure = lower air density (lower engine SHP) (Somerset Dam is higher than Sea Level)

I have a 60HP on a Quintrex. On the bay today I could reach 5500 RPM for 29 Kts at WOT / last weekend at Somerset I could only reach 5000 RPM for 26 Kts at WOT. The boat also feels a little 'sluggish' on the Dam Vs out on the bay.

I plan to use a finer prop up at the Dam as we use the tube and the coarser prop down on the bay.

Just my experiences and hope it helps.

Regards,

Rob

[FNQCairns]

Flex your 50hp is not that big probably 650cc?, if you swapped ratios with the Zuk and kept ypur prop, at 5500rpm you would be traveling at 43kph.

For an established manufacturer the ratio will more often than not reflect the propeller range already sold by the manufacturer than it be the 'best' ratio of which they may need to make another few or line of props that suit.

For the same CC an extra piston gives better torque. The area is huge, if you are looking at a new engine look for the most pistons for the HP rating, look for and choose the engine that also shares its engine with one of higher hp and look for overall total CC in comparison to the competition - very hard to make a poor choice if you do this.

cheers fnq

[Flex]

Thanks FNQ, so you suggest looking at de-rated engines? eg 50hp Yamaha is a de-rated 60hp(996cc4 cylinder).. same with mercury.. Compaired to the suzuki 50(808cc 3 cylinder) which shares it block with a 40hp.

Would you have thought one of these a better performer? in terms of speed and especially efficiency?

[FNQCairns]

Thanks FNQ, so you suggest looking at de-rated engines? eg 50hp Yamaha is a de-rated 60hp(996cc4 cylinder).. same with mercury.. Compaired to the suzuki 50(808cc 3 cylinder) which shares it block with a 40hp.

Would you have thought one of these a better performer? in terms of speed and especially efficiency?

Flex based on that example a person would need a powerful reason (overweight, price) to consider the Zuke over the Yamaha, the Yamaha should outperform it by a good margin...this is apples to apples? both are 4 stroke??

That Yamaha will probably outperform your 50 2st, certainly will as the 60hp, it has the extra piston and the extra cubes so the 60 would be a good choice too.

cheers fnq

[Flex]

Id get the 60hp yammy but my hull is only rated to 50hp. and 115kg's