Proppin da' Wolf... thoughts please

[BogMonster]

Following the sea (lake) trial there have been three props tested on the Vipermax and I need to make a choice, I think I know what I want but just after some comments. For the sake of clarity, we are talking about a Vipermax 5.8m, single dive console plus two extra single jockey seats, A frame, and a 4hp aux, all being pushed along by an XL shaft Etec 150hp, usually with just one or two people on board plus up to 150L fuel.

A lot of folks especially on the Etec Owners Group say that you should prop an Etec to run in the top half of the WOT range, from what I read on tinternet this is to keep the engine revving freely and not labouring.

Roy Bishop who is building the boat says he doesn't agree with this and his point is that the Etec 150 is a very unstressed motor in this application, being the lowest HP version of the 2.6L V6 Etec block, and the largest HP rating for the 5.8 hull, his view is that it can be propped to have long legs for cruising at lower rpm.

Obviously the two arguments are opposite ends of the scale ... so what do I do?

The sea trial tested three props, a Raker 20", a Viper 21" and a Viper 22". Results for WOT were:-

Raker 20" 54mph (47kt) @ 5400rpm
Viper 21" 52mph (45kt) @ 5100-5200rpm
Viper 22" maxed at 5000rpm (didn't ask what speed) so have discounted that.

The Etec 150 has a WOT range of 4850-5850rpm and rated power comes up at 5350 according to the spec sheet which is right in the middle of the WOT range.

I am going to take the Raker anyway as a) it seems suited to the boat, b) it is a second hand one that Roy is offering at a good price and c) I want a spare prop just in case of accidents and cheap is good

So I can have the choice of any prop in the range for the "new" one... the obvious candidates being the Viper in 19", 20" or 21"

I think a 19 will be too revvy and going on the 300rpm per inch rule, would probably be hitting somewhere up around 5800rpm which I don't really want, I have never been a fan of screaming the nuts off any engine as that is usually when conrods start putting in an appearance and we don't want that... so that leaves a choice between 20 or 21. Absolute max speed / rpm is not something I use all that much - and with a big engine on a small boat it is pretty well going to get up and go like sh^t off a shovel whatever goes on it.

I'm thinking a 20 might be a better bet but I have one unanswered question; which one is likely to give the better fuel economy cruising long distance at say 25 to 30 knots? I'm not really sure whether, at any given speed, dropping the revs but loading the engine a little more will increase or decrease fuel consumption?

He did some economy checks (bit difficult as they kept running out of lake...) but observed (I think with the 21" Viper) that the economy at 30mph (26kt) was 20-21lph and best appeared to be around 27mph (23.5kt) at around 17lph which is pretty good at about 0.72L/nm, if it manages that on a regular basis I will be very pleased

I don't suppose it is going to make a huge amount of difference but since I have the choice, I'd just like some opinions please...

Ta

[jwalker]

Easy. From your figures you're bang on max power at 20" with a virtually unloaded boat. Add anchor crew, fuel and all the other bits n pieces and you'll fall below max power. So, your 20" will turn into a slightly high pitch cruising prop and a 19" will be on the button.
If you're travelling light you'll still be within the engine rev range, if heavy you'll have enough bite to climb the backs of steep waves and not be labouring at lower speeds in poor sea conditions. On the fun side, your hole shot will be worth opening the throttle for.

Make sure they test a 19" before they post your boat.

[Hightower]

19" here as well You won't believe the weight you add with putting all your gear aboard. Ropes, Anchor, toolkit, spare prop, spare 2 stroke oil and the likes. Add to this your daily gear too, like fishing gear, crew, sheep and cool box with the tinnies in, yeah definitely a 19"er. Get Roy to chuck some sandbags in and do a speed run.

[martini]

I'll share one of my observations on playing with props, obviously the actual figures will mean nothing to you as it's a completely different setup.

I'm using Merc revolution 4 bladed props:

19"
WOT = 5400 rpm @ 44 knots
@ 30 knots cruising speed 4000rpm consumption = 2.8 litres/Nm

17"
WOT = 5700 rpm @ 41 knots
@ 30 knots cruising speed 4250rpm consumption = 2.6 litres/Nm

Obviously if this was a racing boat I'd be using the 19's but it's not. For any given speed the 17's are revving higher but burning less motion lotion and I bet theyre putting less wear on the engines.

It really doesn't matter if you can over-rev the engine at WOT, just don't use WOT and it won't over-rev!

By slightly under-propping the boat you can save fuel as well as reduce engine wear, fair enough you may lose a couple of knots at top end but who cares?

[Polwart]

Originally Posted by martini

By slightly under-propping the boat you can save fuel as well as reduce engine wear, fair enough you may lose a couple of knots at top end but who cares?

Especially if you'll be the fastest rib for 1000 miles in any direction anyway!

[BogMonster]

Mmmm all very interesting thoughts, ta.

I don't think the typical operating weight for me will be a lot different to the test weight - I think they were 2 up in testing which is probably the same as me (I am perhaps carrying one or two pies ), plus an anchor/chain, aux, and some extra fuel. I don't carry a lot of other crap on the boat. At the present time I don't tend to load it up with people, there is either just me or two of us and on very odd occasions a third person, on the odd occasions I get roped into carrying a crowd (maybe once a year) they will be sitting on the tubes so flying around like a loon is not going to happen, if you fall in the South Atlantic you're usually dead in about five minutes so tipping the passengers in the oggin is not recommended!

Chris Stevens, running a heavier 6.5m Vmax with the same engine, told me he gets 5500rpm on a 19" Viper, which would suggest that with a shorter and lighter boat it is going to be well above that. Having thought about it some more since I posted, I don't think I want a prop that does not reach at least the rated power RPM of 5350, a 20" Viper should do that and a 19 would be well over, and the responsiveness might be quite fun

Unfortunately I don't think there will be time to do another test run before despatch. I'll have to mull it over and see but the fact that a lower pitch might actually improve fuel consumption in the cruise is something I'd not really considered. Decisions decisions

[Hightower]

I've got a 19" on my 6.2 and it's OK but it only revs to 4900rpm, but I'd love to try a 17" one on there and see what that's like. Of course it's a different engine of that of your one Stephen and I believe the bottom gear ratio is different too. My WOT operating range is from 4750 to 5250.

So anyone got a 17" Viper LH that I could try

[BogMonster]

Originally Posted by Hightower

I've got a 19" on my 6.2 and it's OK but it only revs to 4900rpm, but I'd love to try a 17" one on there and see what that's like. Of course it's a different engine of that of your one Stephen and I believe the bottom gear ratio is different too. My WOT operating range is from 4750 to 5250.

So anyone got a 17" Viper LH that I could try

What are you running - Etec or something else?

[Hightower]

Originally Posted by BogMonster

What are you running - Etec or something else?

It's an Evinrude DI

[GordyP]

Ehh, did you say you were getting a good price on the prop? You get a new stainless prop as standard with a new e-tec, you should NOT be paying extra for the prop.

Hightower, i have a spare rh 17" viper if you want to try it.

Edit - Just read your post properly and missed the raker is a spare. Ignore me.