Help propping boat

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Post Options Post Options   Likes (0) Likes(0)   Quote Bounty Hunter Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jul 2019 at 7:29pm
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Titanium
Titanium
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Joined: 14 Nov 2002
Location: Russell
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Originally posted by MATTOO MATTOO wrote:

So in reading this post I'm evaluating the value.

To be honest, cost of changing a prop here doesn't show a lot of value.

I think in small boats the benefits are minimal.

That comment is based around style and frequency of use of your vessel.

In the main, a alloy standard prop is most suitable for a Harbour or gulf or day out boatie or fisherman.

If your engine hours per season extend above 300, you crosss bars regularly or beach launch, or troll or have large hours above what I have suggested than there is little point in a change.

If you do, go stainless, and do your calls after a 300 recorded hours so an educated decision can be made.

Otherwise your pissing money and time away.

i think this is an over-simplification of the potential benefits of a well match propeller. 

no doubt that buying replacement s/s props is expensive, but the difference between a poor match and a really appropriate one can be massive, with significantly improved speed/efficiency and also much better boat handling.

it appears that boats are often bought - either new or second hand - that are frequently poor performers because of an inappropriate propeller - and i think that the cost of a new wheel is soon overshadowed by the enjoyment of better hull performance. 

i think the best thing we can do is educate and help promote that dealers of new rigs have access to a 'pool' of propellers from outboard agents which are available for rigging, and that buyers should do some water testing and if necessary request changes before they accept that the purchase of a new boat/motor is final and complete. 
No disintegrations!
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Post Options Post Options   Likes (0) Likes(0)   Quote MATTOO Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jul 2019 at 7:59pm
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Titanium
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Cathchelot, Bounty Hunter.

What insightful comments.

I think you expand the conversation about choices and consequences.
And identifying the need of sales staff or boat retailers as being good is a phalisy (poor spelling me thinks).
They need to do better.

I suggest alloy as a cost alternative for a person beginning.
Yes they do damage easily.
But this maybe a more cost effective tool for a mistake in the beginning of a boaties first steps.

Stainless with formats as "Steps" suggests are the more ideal alround tool for performance and economy.

I suppose I'm trying to balance my comments to those who maybe reading a post like this.
Lower demonitator so to speak.
Just cruising in my now sweetas pimped out Southern 755 HT0!
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jul 2019 at 9:29am
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Titanium
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All good comments.
 From experience, a good proportion of boats that have engines mounted correctly are propped ball park.. that is to go spend $600 + on a new stainless doesnt warrant the capital cost.
 Those that are not mounted correctly are very inefficient and usually propped to that (in effect) extra 100kg or so of drag.
The motor gets mounted correctly, some +ves are seen, but there are significant further benefits even on lower use boats.
These are generally in the marginal worth the capital investment...
And this is where I have found roughly what loads, power boat weights, speed and application where the alloy still works..but at high speeds rarely used, has a small change..not significant enough to go stainless.

The SS 4 blade had on the 115hp, that took some 2yrs watching trademe.. picked up for $150/200.. in the mean time was using a very similar 3 blade picked up for $50/75. And before that a $50 alloy from my local service center.
The differences in economy where not significant, say $10 a trip...but the difference when hit chop, espec big chop huge.

The performance of each of those I was able to calc the difference from accurate data of the previous prop(s) before purchase.

There are heaps of good 2nd hand REASONABLY priced props on trademe and the local service center shelves... even getting something closer to target for $50 the capital cost can be picked up in 3 or 5  day trips easy.

The comments above on stainless  for shallows reefs etc.. yep for sure.
Give a rock / ramp at anything above idle you suddenly load up gearbox crank etc...new gearbox there goes $2500/ 3000 easy.

I clipped a gravel bank on the Waikato last Xmas.. at a little above idle...the rock/ petrified bit wood .. whatever it was, caught between the prop and tip of the skeg and snapped 1/2" of the tip off.... The boat doesnt swing as wel at anchor or drift when use the engine as a rudder to keep boat in right direction....and the sweep of a turn at cruise is a little wider.

If your boat gross weight is around 1200kg on the water.
Cruise speed 3800/4000 rpms (rpms not critical) is around the 32/35 mph.. alloy twist is not significant...load the blades up more and you will see the difference in data... but above cruise speed, for general use boats, that is not where the boat is being used most of the time.
 Get up into 6m hulls, 1500kg gross weight on the water.. the load to move that weight on the plain even at slow cruise..there is significant twist in blades.

Generally one can go slightly smaller diam with a SS prop as these tend to have significantly more cupping (more grip per diameter)..  this is a fine tune thing, and with general use boats we are using  such fine tuning is not significant enough to warrant the capital cost.

One can not determine a correct or close to correct pitch, design, blades etc without knowing from accurate data from slow cruise thru to WoT, on flat water at best trim for each speed, the slip of the current ball park prop.

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