how fast should it go

[breezeblock]

I've got a brig 5m rib with a honda bf75 on it the revs i think go up to about 5100-5200 with the throttle pushed way forward as far as it will go and the most i could get out of it is 32 knots on the gps this was going with the tide going out on loch etive does this sound ok as I've read that these engines should be 6000 rpm max i don't know the size of the prop

[jeffstevens763@g]

Size should be stamped on the prop looks like you could go up in pitch

Cheers

[Pikey Dave]

Originally Posted by jeffstevens763@g

?........ looks like you could go up in pitch

Cheers

Down surely??


Sh1t happens

[paintman]

If it's this one then your WOT range is 5000-6000 rpm so you're right at the bottom end.
Honda BF75 and BF90 Outboard Engines | 75 and 90 hp 4 Stroke Motor Specs and Features

Purely as an example, if you were achieving your 5200 with a 23" pitch prop then going to a 21" pitch should increase your WOT rpm reading to around 5500/5600 & a 19" pitch to around 5800/6000. Each 1" pitch change will change the rpm by 150 to 200 rpm.
Bear in mind that you should carry out any test runs with the boat normally loaded. You do need to know what RPM you are achieving & what pitch the prop is.

There are a number of prop calculator sites on the net so you might want to find one & enter your details into it to see what's recommended.

[whisper]

Originally Posted by paintman

If it's this one then your WOT range is 5000-6000 rpm so you're right at the bottom.
Honda BF75 and BF90 Outboard Engines | 75 and 90 hp 4 Stroke Motor Specs and Features

Purely as an example, if you were achieving your 5200 with a 23" pitch prop then going to a 21" pitch should increase your WOT rpm reading to around 5500/5600 & a 19" pitch to around 5800/6000. Each 1" pitch change will change the rpm by 150 to 200 rpm.
Bear in mind that you should carry out any test runs with the boat normally loaded. You do need to know what RPM you are achieving & what pitch the prop is.

There are a number of prop calculator sites on the net so you might want to find one & enter your details into it to see what's recommended.

^ +1

You also need to be trimmed out correctly, be sure to test this at the correct trim before buying new prop. also check if there's water stuck in the hull or crap on the hull etc

[breezeblock]

thanks for replies as a matter of interest what do similar set ups achieve the spec that was posted is the same as my outboard it does say in the spec that max power is at 5500 rpm so i think i should be reducing the size/pitch of prop to get the rpm up cheers

[paintman]

Need to know what size & material prop you currently have fitted.
Look for one of the online prop selectors, you'll see that they give a number of different targets - e.g acceleration, good allround etc - & prop recommendations can vary.
This is Mercury's - bear in mind it will only list the props they make.
https://www.mercurymarine.com/en/us/...tor/#/step-one

[whisper]

From a very wide 5.45m with a merc 75 four stroke we get 30 knots 4 up with a 17" it came with a chewed up 15" but it was over reving.

Also changed the fuel pumps and fuel system from the tank to the pumps, cleaned and balanced the 4 carbs.

We have antifoul that will come off one day when I feel like it. But first I'm going to remove the Dolphins on the leg as they seem to do more harm than good.

A 30 knot 4 seater is perfect for us even though I could go up to a 115hp, however if I really wanted some more power I could change the carbs as that's the only difference between the 75 and 90? But the fuel consumption is excellent with the 75, possibly the most fuel efficient boat I've owned




www.customrib.co.uk

[jeffstevens763@g]

Originally Posted by Pikey Dave

Down surely??


Sh1t happens

Stand corrected got my up in revs mixed with pitch yes down

[MatFromPoland]

Your engine has gear ratio 2,33
RPM 5200
Slip 10%

So it looks like you can have 19" pitch prop:
RPM 5200
pitch prop 19
gear r. 2,33
slip % 10
knots 33

If you change to 17" and reach 5500 RPM you can have lower top speed:
RPM 5500
pitch prop 17
gear r. 2,33
slip % 10
knots 30,57811159

If you go easy to plane not sure if there is a reason to change PROP.
Check water in bilge/under the deck.
Inspect fuel line (for example change fuel filter if you have any)

33 knots is really reasonable high speed.
In this boat size on the Sea you use 25 knots or less

[9D280]

Also bear in mind the diameter of the prop also affects the RPM......


FWIW, I'm 60Hp, 5m and the difference in literally a tap or two of the trim switch can make or loose 4 knots.

E.G on Monday I had it running at something like 4800 rpm, getting about 20 knts. I trimmed up a tiny amount, didn't move the throttle, and next thing I'm doing 5300rpm, 25 knots and throttled back a bit for a quieter / more economic run!



If you were going with the tide on Etive shouldn't you have been doing about 108 knots?

[breezeblock]

i remembered to check the prop when i was over at the boat doing some bits an bobs to it and the prop is 12.5 x 19 so the 32 knots i was getting out of it isn't that far off the mark to be honest i was quite happy with this as its fast enough

[Iankristy]

To check speed accurately you need to go in opposite directions and take the average 👍