North Australian FISH FINDER TM forums

Bh marine or George at nt unique

Scottie5.3 wrote:Bh marine or George at nt unique

Ok Scottie I'll ring them tomorrow, who should I speak to?

Byron at bh, he will probably ask you to bring it in. Or George at nt unique, he's a knowledgable bloke and probably know off hand what might work

Ah ok, that won't work then as I live in NQ haha

I got your pm bloke, mine is a basic savage 4.45m tinny, and I mean basic, no gunnels with not of much of a floor so its light. You look to have a very similar mounting position with the cav plate level with the bottom of your hull which I believe is correct. My foil is very similar to yours and im running an 11.1x13 solas 3 blade prop which after looking at it is in bad shape lol.

Based on the specification (http://marine.honda.com/outboards/motor-detail/BF40-50) the maximum speed you could attain is 57 km/h with a 12 inch pitch assuming no loss to prop slip.

Max Speed = Max RPM * 60 seconds / Gear Ratio * Pitch * 25.4 / 1,000,000
in your case
Max Speed = 6500 * 60 / 2.08 * 12 * 25.4 / 1,000,000
Max Speed = 57 km/h

A 13 would give you about 62 km/h and a 14 about 66 km/h assuming that it's not getting to big. 50 km/h @ 4000 rpm is mathematically impossible with your engine and prop combination, 35 km/h would be more realistic assuming no slip. Factoring in some slip say 20% (at full throttle you're getting about 35%) the boat is only travelling around 28 km/h, so expecting the boat to plane may be unrealistic.

There's an online calculator at http://www.go-fast.com/Prop_Slip_Calculator.htm but it's in mph so multiply by 1.6 or try http://www.mercuryracing.com/prop-slip-calculator/.

Dan

Hard facts right there. I would of thought prop slip would be under 10% once the boat is on the plain but I've been wrong before!

qiksix wrote:I got your pm bloke, mine is a basic savage 4.45m tinny, and I mean basic, no gunnels with not of much of a floor so its light. You look to have a very similar mounting position with the cav plate level with the bottom of your hull which I believe is correct. My foil is very similar to yours and im running an 11.1x13 solas 3 blade prop which after looking at it is in bad shape lol.

Yeah just grabbed a 11.1 x 13 off my mates boat and testing it this arvo. Cheers for the info mate

dannett wrote:Based on the specification (http://marine.honda.com/outboards/motor-detail/BF40-50) the maximum speed you could attain is 57 km/h with a 12 inch pitch assuming no loss to prop slip.

Max Speed = Max RPM * 60 seconds / Gear Ratio * Pitch * 25.4 / 1,000,000
in your case
Max Speed = 6500 * 60 / 2.08 * 12 * 25.4 / 1,000,000
Max Speed = 57 km/h

A 13 would give you about 62 km/h and a 14 about 66 km/h assuming that it's not getting to big. 50 km/h @ 4000 rpm is mathematically impossible with your engine and prop combination, 35 km/h would be more realistic assuming no slip. Factoring in some slip say 20% (at full throttle you're getting about 35%) the boat is only travelling around 28 km/h, so expecting the boat to plane may be unrealistic.

There's an online calculator at http://www.go-fast.com/Prop_Slip_Calculator.htm but it's in mph so multiply by 1.6 or try http://www.mercuryracing.com/prop-slip-calculator/.

Dan

Hey mate, they just what's got off boatsales review page. Well I wasn't even getting 50km/h so try this other prop and see how it goes.

Pretty cool equation you got going up there!! Very helpful

Well tried the 13p prop this arvo and it made no difference, didn't give me anymore speed or anything except it knocked about 300rpm off full throttle bringing it back down to around the 6200rpm range which is still more then it should be. Top speed I got was 48km/h and that was on a flat river. So what's next lol

Whats the bottom of your hull look like, any big dents?

Na all normal, it had a hull side repair but that area is out of the water and got certified

I see your in Townsville have alook on buy swap sell boats Townsville on Facebook there ais good private mechanic advertisers on there might be worth a chat comes highly recomened or take a trip to innisfail see Joel at innisfail marine he's the one that sorted my boat out or if ya want to make a fishing trip to in Karumba mechanic there is very knowledgable

And are you using gps to check speed or just reading the engine gauge not familar with honda but if anything like merc the readout from the engine never reads accurate reads anywhere from 5 to 10 ks slower when going flat out depending on conditions

This has actually got me interested, ive just done some calcs on my larger boat to work out prop slip. Turns out dannett is about right. Ive got a 26ft glass boat with a 350 mercruiser with a bravo 3 sterndive with twin duo props. Full noise at 5000rpm I get roughly 70kph. The calculations actually give me a speed of 92kph. That's about 20% prop slip at full noise.

Your boat no doubt being heavier than my bare tinny makes me believe that you could be getting a bit more slippage than myself. Another good indicator is the fact you dropped 300rpm with a 13p prop. I reckon your little 50hp is using all of its 50hp to do what it is now, it's on it's limit.