Prop size for Suzuki DF70.

Jackwabbit · 18 May 2004, 06:54

Can anyone help ?
Back in January our diving club picked up its first RIB a Humber Assault 5.5m, up until now we have always used inflatables. This boat came with a Suzuki DF70 4 stroke engine with a 13½ X 17 prop. While using this boat for the RYA PB part2, we were able to get 30knts at 5500 rpm with only 4 people on board & no diving gear. Now we have started to use it for diving (with 7 people & 6 lots of diving kit) it is taking a long time to get up on the plain & the max speed is 20-24knts with max revs are 4700-5000rpm. Now I know this is over propped & the engine is not reaching its full throttle operating RPM which is 5200-5800, and I don’t want to do 30knts with this small boat full of divers (24knts is uncomfortable enough when it get a bit bouncy) what I do want is for the engine not to be stressed all the time & for the boat to get up on the plain quicker.
So can anyone recommend what prop size we should have & where is the cheapest place to get one ? (our club is a bit skint for a while after buying this boat)
I have asked the same question to Humber & they took all the information & said they would get back that was about a week a go now (I’m not holding my breath)
Now all I have to do is convince the committee we need to spend more money

tim griffin · 18 May 2004, 07:16

have you tried talking to the engine manufacturers, they might put you on the right track, there are quit a few engineers on here that will point you in the right direction,and you could always sell your old prob on ebay or rib net to recover part of the costs of a new prop, it seems a bit of a black art to me regarding the correct prop for the boats set up and use, i have a 21 on the back of my rib and am switching to a 19 because of same problem as you,
will be interesting to see the difference when she is under load.
regards tim
www.griffmarineservices.co.uk

Neil R · 18 May 2004, 09:10

How about trying one of those hydrofoil thingies - doelfin seems to be the market leader - one size fits all and I think about £50 all in.

I've had a similar experience with a DF90 - I tried a 2" lower pitch prop thinking it would allow the engine to rev a bit more - it did - but the overall result was pretty much unchanged performace, but with a higher revving engine! So I agree it's a black art. At least I now have a spare ...

I've not ordered my fin yet, but there seems to be enough positive vibes on here when I did a search to make me want one. And if it doesn't work, at least it should be more saleable than a specific size of propeller?

I bet you're going to tell us now it already has one ...

Jackwabbit · 18 May 2004, 09:48

Quote:

Originally Posted by Neil R

How about trying one of those hydrofoil thingies - doelfin seems to be the market leader - one size fits all and I think about £50 all in.

I've had a similar experience with a DF90 - I tried a 2" lower pitch prop thinking it would allow the engine to rev a bit more - it did - but the overall result was pretty much unchanged performace, but with a higher revving engine! So I agree it's a black art. At least I now have a spare ...

I've not ordered my fin yet, but there seems to be enough positive vibes on here when I did a search to make me want one. And if it doesn't work, at least it should be more saleable than a specific size of propeller?

I bet you're going to tell us now it already has one ...

No we don't have a hydrofoil on our engine, this was one of my suggestions & at less than £30 the cheapest option, but our club has quite a few old fossils who think they are always right, and their opinion is its only a gimmicky thing that couldn’t make any difference.

Mind you its only taken our equipment officer & myself nearly 4 years to get them to get a RIB (they kept saying “why do we need one of those we have always used inflatables since 1967 without any problem, why change now?”
We had to point out that none of them were still running around in Hillman Minx’s or Ford Prefects, they all had cars that were less than 3 years old, so why did they change if the old cars were ok in 1967.

Searider · 18 May 2004, 10:29

Speak with Steel Developments - propellers are their business and they will give you some good advice on what the best solution might be.

post as steel dev on this forum or telephone 0208 874 7059.

Spoke with them this morning about a s/h ss prop I have just bought (not from them) thinking that I might have a bush problem (expense) but might just need a balance (less expensive)

Ribald · 18 May 2004, 10:42

If you are coming to RIBEX, Steel Developments will be there with a selection of props for people to try out on their boats. I'm sure you could whistle up the equivalent weight in bodies and diving gear from those attending to ensure the loading is right

( for the cost of a beer or two)

We had to point out that none of them were still running around in Hillman Minx’s or Ford Prefects, they all had cars that were less than 3 years old, so why did they change if the old cars were ok in 1967 - I use the exact same analogy when telling people that their heating boilers are stuffed - works every time

Jackwabbit · 18 May 2004, 11:14

Thanks for all your help.

I have just spoken to someone at Steel Developments, they confirmed the prop is too large, and we are not reaching maximum HP for the engine. They suggested going down to the next size 13½ X 15, this would increase the engine revs by 500RPM and bring it back into the MAX HP band. This would allow the boat to get on the plain quicker without loosing hardly any top speed.

All I’ve got to do now is persuade the committee to cough up another £106 for a new prop.

roohairy · 18 May 2004, 18:55

Speak to some of the people who do prop repairs, I've got them to exchange props for me before - If yours is in perfect condition and you get a recon one it shouldn't cost you anything like £100.