Help chosing the correct prop for a slightly unusual speedboat.

[Crunk]

Hello all,

Over the weekend I took my boat out for the first time, I bought it needing a complete restoration which I completed over winter.

The boat itself is a 1960's Fletcher Arrow 120, it's around 11.5ft in length as has a very flat bottom unlike more modern Fletchers.

I'm using a Suzuki DF20A outboard which seems to be working well, it gets on the plane at around half throttle at 3,000rpm and 18knots but increasing to full power brings it up to 23knots but only 3,500rpm (full throttle operating range should be between 5,300-6,300 according to Suzuki) which to me would indicate it's drastically over propped.

The prop I'm using is what was fitted from new, a 9.25 x 12, this is also the coarsest pitch they do. Given the RPM I'm seeing at WOT, what pitch should I drop down to? I was thinking a 9 or 10?

Thanks in advance!

 

[paradave]

No idea on the prop but a lovely looking little boat.

 

[ChromeDome]

Let me have your ready-to-go GW and I'll suggest a pitch.

 

[Elessar]

Hello all,

Over the weekend I took my boat out for the first time, I bought it needing a complete restoration which I completed over winter.

The boat itself is a 1960's Fletcher Arrow 120, it's around 11.5ft in length as has a very flat bottom unlike more modern Fletchers.

I'm using a Suzuki DF20A outboard which seems to be working well, it gets on the plane at around half throttle at 3,000rpm and 18knots but increasing to full power brings it up to 23knots but only 3,500rpm (full throttle operating range should be between 5,300-6,300 according to Suzuki) which to me would indicate it's drastically over propped.

The prop I'm using is what was fitted from new, a 9.25 x 12, this is also the coarsest pitch they do. Given the RPM I'm seeing at WOT, what pitch should I drop down to? I was thinking a 9 or 10?

Thanks in advance!

I thought it was about 200rpm per inch change. Which sounds like a much bigger change than you are suggesting. I'd be concerned there was something else wrong in the engine stopping you getting to those revs.

First question to ask is - is your rev counter accurate? Get a hand held digital rev counter to test.

I'm sure you'll get lots of ideas here. But you could also ask someone like hamble props who know their stuff, they'll have an idea at least.

 

[Crunk]

Thanks,

I was reading circa 200rpm per inch too, however I also recall seeing somewhere that on smaller engines the difference is greater (closer to 400rpm per inch).

The engine does seem to be functioning well, the DF20A tends to go into a limp mode and flash a CEL if there is anything untoward which it's not doing, The Rev counter does appear to be accurate, it sits at 850rpm on idle which is exactly on spec and the RPM increase (to the ear at least) seems consistent.

Another though I've just has is perhaps I'm trimmed in too far and when it's getting on the plane it's pushing the bow down more and hitting a wall. It certainly doesn't lift the bow much at all when getting on the throttle, so perhaps I should move the trim position out one position first.

 

[Bouba]

Find a shop who will let you borrow a prop before committing yourself

 

[Crunk]

Let me have your ready-to-go GW and I'll suggest a pitch.

Thanks,
I'll need to find a way to weigh it and let you know.

 

[ChromeDome]

For speed boats the calculation is:

A given number of horses can push a certain GW to a max speed IF:

The engine reaches the max at WOT range (5300-6300 in your case).

Example: 20 horses can do 26 knot on a flat hull if the GW is 400 kg. Given the gearbox ratio of 2.08 on the Suzi, it needs a 12" pitch prop to end at the mid (5800) of the rpm range.

Reduce weight and or add HP go faster. May have to change prop to meet the rpm range, though.

If you know the current pitch and reached rpm, you can calculate the needed change in pitch to reach desired rpm (if not already there) as each inch increase of pitch will take 200 rpm off - and vise versa.
Since (almost all) props are produced in 2" steps, the least change you can get is 400 rpm
All by rule of thumb, only testing will show the reality.

 

[Alicatt]

The DF20A share the same mechanicals in the engine as the DF9.9 and DF15, the difference is in the ECU and a restrictor in the exhaust for the lower horsepower engines, they restrict the maximum RPM that the engine can achieve.

If you have no luck with changing the prop it may be worth looking at the ECU is the correct one and if there is a restrictor/spark arrestor in the exhaust, the DF9.9 tops out about 3600rpm.

 

[Crunk]

The DF20A share the same mechanicals in the engine as the DF9.9 and DF15, the difference is in the ECU and a restrictor in the exhaust for the lower horsepower engines, they restrict the maximum RPM that the engine can achieve.

If you have no luck with changing the prop it may be worth looking at the ECU is the correct one and if there is a restrictor/spark arrestor in the exhaust, the DF9.9 tops out about 3600rpm.

That's interesting to know. I'll bear that in mind. Thanks

 

[Crunk]

For speed boats the calculation is:

A given number of horses can push a certain GW to a max speed IF:

The engine reaches the max at WOT range (5300-6300 in your case).

Example: 20 horses can do 26 knot on a flat hull if the GW is 400 kg. Given the gearbox ratio of 2.08 on the Suzi, it needs a 12" pitch prop to end at the mid (5800) of the rpm range.

Reduce weight and or add HP go faster. May have to change prop to meet the rpm range, though.

If you know the current pitch and reached rpm, you can calculate the needed change in pitch to reach desired rpm (if not already there) as each inch increase of pitch will take 200 rpm off - and vise versa.
Since (almost all) props are produced in 2" steps, the least change you can get is 400 rpm
All by rule of thumb, only testing will show the reality.

Thanks, interesting stuff!
It seems the Suzuki DF20A props have quite a range of pitches from 8,9,10,11 to 12.
I'm not too hung up on the top speed, I didn't want to be labouring the engine though, although 30knot's would be nice! :-D

It sounds like I really need to get the boat weighed.

Cheers

 

[ChromeDome]

Thanks,
I'll need to find a way to weigh it and let you know.

Boat dry weight (try Google) + engine + fuel + gear, stuff, equipment + crew.

Tends to surprise, when you add it all up.

 

[Crunk]

Boat dry weight (try Google) + engine + fuel + gear, stuff, equipment + crew.

Tends to surprise, when you add it all up.

The factory weight was 113kg, however in the rebuild I've used a thicker transom, and added the weight of a marine ply floor.
The motor is 54kg and I'd say all the extra ply, resin and glass would easily be another 50kg, running a 42L tank up front (31kg) and I weight 67kg, my son is rought 25kg so as a rough guess around 350kg on the water.

 

[Crunk]

The factory weight was 113kg, however in the rebuild I've used a thicker transom, and added the weight of a marine ply floor.
The motor is 54kg and I'd say all the extra ply, resin and glass would easily be another 50kg, running a 42L tank up front (31kg) and I weight 67kg, my son is rought 25kg so as a rough guess around 350kg on the water.

Doh, so many typos! Sorry, I was multitasking (badly it would seem!)

 

[ChromeDome]

The universal excuse for writing rubbish is AI.

20 hp will push 350 kg to 27,8 knots.

Interested to know the rpm at wot and the number(s) stamped on the prop hub somewhere (commonly between the blades.

 

[ontheplane]

If it was horribly over-propped though would it get up on the plane as you suggest? I too question if it’s the prop or whether there is something else wrong.

I had a maximum 1700 with a 75 on it, and that wouldn’t plane when I bought it. Had to drop 3” of pitch before it would, then it went up to 5000rpm happily.. could have gone down another inch or two probably. But point is, if massively over-propped it would bog down and not get to 3000rpm at all

 

[ChromeDome]

The calcutator has it right. But only a test will tell.

 

[Crunk]

Thanks for the replies all! What a wonderfully helpful community this is!

I've checked the ECU numbers and it's defintely the correct ECU for the 20hp

I've ordered a diagnostic cable so I can monitor what's actually happening and if there are any fault codes.

I've also ordered a 9" pitch prop and hope to test it this weekend.

I'll update with the results.

 

[ChromeDome]

I've also ordered a 9" pitch prop and hope to test it this weekend.

I'll update with the results.

That is going to be interesting.
I'd expect a good hole shot, reduced top end and a tendency to rev beyond the recommeneded range.

Looking forward to your findings.

 

[Crunk]

That is going to be interesting.
I'd expect a good hole shot, reduced top end and a tendency to rev beyond the recommeneded range.

Looking forward to your findings.

Currently I'm sitting 2500rpm below the optimal at WOT with the 12" pitch, will be interesting to see.