Help propping boat

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    Posted: 18 Oct 2018 at 4:13pm
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Hey to all.  I am looking foward to the summer with baited breath, long hours of trolling and gamefish to boot, but the cost... oh dear.

ATM my Suzuki df115 is running a standard prop and a permatrim.  I cannot fault fuel consumption for general fishing driving, WOT brings 34knotts at 6000rpm but trolling for gamefish is pretty tricky.  

At 2-2.2k RMP the boat runs really well cheap etc at 5.5 knots 
2.4k RPM 6knott
2.6kRPM 6.5-7knots its starting to suck fuel
2.8k RMP 7-8 knots its thirsty AS
3k RMP on plane 12-15knots seas state dependent.

I have read abit out different propellers and was trying to get some better economy lower down.  Ideally I would like to run around 7 knots and not drink my fuel so the RPM needs to be 2.2-2.4max

 4Blade?
Bigger pitch?

Help?
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Stainless prop as they do not flex?
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Alloy props flex under load.
 Load means going very fast and or pushing a very heavy boat/barge.

Propping for a plaining hull starts at boat weight and HP at the prop.
"Std Prop" no such thing...a boat is proped to the above and end use... end use maybe towing skiers, or general use, or trolling.
 If well powered then one has enough reserve power at any given rpms to be able to uase an all round prop selection.
 Min powered or less, then people have a prop for 'pulling the kids around, and another for general fishing.

Then we throw another factoer in.. trolling, thats not on the plain...which then means the hull is basically a displacement hull.. totally diferent kettle of fish.

So need some info.
 1/Current prop pitch and diameter/ number blades.
2/ Is the motor correct height?
 THIS IS VERY IMPORTANT.. it is a waste of time going any further if it is not...
 At fast cruise speed, flat water, best trim for that speed, the bottom of the cavitation plate is in the water, top surface has a few droplets running down the surface.
3/ Motor HP    (115 hp) 
4/ motor model/ gear box ratio. (2.59)
 5/ Manufacturer WoT rpm Range (5000 to 6000) Unless a a race boat, this should be boat fully loaded, midway or slightly below ...
 6/ Total gross weight on the water 
And/or WoT speed/ rpms at best trim for that speed on flat water.    (34kt @6000rpms calc to approx 1350/1400kg)
Rpms and speed at trolling
7/ Rpms and speed, flat water, starting around 3500 rpms in approx 500 rpm increments thru to WoT.
 IE set rpms at 3500..then bring trim up to just before cavitation and/ or porpoising... note rpms and speed.
 Then set to 4000 rpms, re adjust trim.. slightly higher as above , and repeat each time. 
8/ Boat make/model.

 Ball park min powered boat between approx 4.5 and 6m will have a WoT speed around 42mph 18/19" pitch prop.. and a well powered 46 to 48 mph..19/21" prop When engine height is correct and no extra drag stuff like a permatrim, or tabs down.

If the boat is min or well powered you will not need the permatrim.. the permatrim in the water at trolling speed adds a lot of drag../ fuel use. A prop with more rake is the correct way to lower the bow/ lift the stern if need be.

Going on current limited info. assuming engine correct height  Prop correct diameter etc
115 hp 34 knt (39 mph) 6000rpms
2.59 gearbox 

Total gross weight on the water 1350/1400kg
you have a 19  pitch prop assuming the grip/ diameter is correct
 A 4000 rpm cruise speed approx 24 mph.
 
 I do suspect that the prop diameter/ cupping maybe incorrect and pitch maybe too high.
 Better data required. 




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The prop isn't determining your fuel consumption. Your hull length is.A ~6m boat has maybe a LWL of 18ft, could be less. That gives a theoretical displacement speed of around 5.5knts. In other words, once you go past that speed the hull is trying to climb onto the plane or at least push an ever increasing amount of water. That typically means staying around the 5.5 - 6knt range for max efficiency when trolling. Doesn't matter what you do with the prop you can't 'fix' this. It is possible that by going down in prop size and getting the engine running closer to it's correct loading curve that you might do a little better but I don't think you will find a magic bullet unfortunately. To efficiently troll at 8knts you need to be in that 35 - 40ft hull length range. Tagit at 44ft trolls at 8.5knts on 15lph of diesel. You would probably get close to the same cost trying to troll a 6m outboard boat at the same speed. It's all about hull length at those speeds.
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correct"tagit"hull length + planning hull.  at 6/7 knots more hull in water displacing more water. trim internally.ie move weight forward to lift stern a bit.easier to push water than to drag it. Had it explained a number of years ago by a designer to me,a yacht going downwind in the light,weight forward and push water,better and easier than drgging it by stern. So can only guess same principle would work with planning hull trying to do what a displacement hull is designed for at lower speeds.
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One thing to be very aware of with the Suzuki's is they have the offset drive shaft with double reduction. They generally run at least one size bigger prop than an equivalent 4 stroke from another brand. You will get economy but as Tagit says length is a factor especially with sonething like a Surtees as they are a HP hungry hull to push.

Pm me if you want some help Suzuki wise.

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Here is a pic of the prop and specs.

It looks like a 3 up close
X 14 x 19


Outboard is correct height 5 mm water max over the cav plate when planning.

17 to 18knotts at 4000rpm flat seas


I am interested about the hull size etc. And it does make sense. My last outboard being a Honda 90 sat at 3330 and did 8.5 knots all day long without using loads of fuel... hmm still should probably get a better prop

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What year is the Suzuki did you get it new.

What hull is it pushing?
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I am interested about the hull size etc. And it does make sense.

 I mentioned in my post.. when off the plain you are running baically a displacement hull.. that Tagit expands on very well
 I know my trailer planing hull stuff, displacement hulls a totally different "kettle of fish".. other than the basic of the basics.. I dont know.
 Planning is about weight, HP.. more hp the faster you go.
 Displacement is length, and there is a point that adding more hps the hull doesnt go any faster. Rather over simplified 
Very different..
 What I have found thu, is if a plaining hull is set up well for general use, and is well powered, and good grip (slip%) for general use, going a little on the low side of targeted slip, the troll/ displacement improves significantly.
 Min or underpowered, fuel economy hurts on the plain and trolling numbers as there is little or no reserve power at that rpms.
 Simply throwing on a larger prop drops the slip, better econony, but if not well powered issues like oiling up, continuous loading of the power head as all bad news in the long run.
 Same principle for a tractor bulldozer airplane etc.
 Which Tagit also explains above.

last outboard being a Honda 90 sat at 3330 and did 8.5 knots all day long without using loads of fuel... hmm still should probably get a better prop 

You have to establish if the current prop is incorrect 1st.
 There are some questions above / data that required..
 And best to do so without the permatrim and the cavition plate as I decsribed..
 Having the cavitation plate on and under water at cruise will add a good 100 / 150 kg to the effective boat weight in drag. 
 based on data, that is suspect not gathered at best trim for that speed, as described in my post above..
 
115hp
2.59 :1 ratio
 34kn@6000
17.5 kn@4000
 the total gross weight on the water including permatrim drag , and engine marginally low is around 1350/1400kg
 Thats a good 150kg  heavier than my fully loaded 5.5m old commander or a loaded 5.5m rayglass..
 What boat is this?
 The slip numbers if a 4.5 to 6m boat sound good..@ woT but rather high at cruise...indicating poor triming, drag etc.. should be cruising around 20/21knts @4000 rpms.

This will then increase the slip when trolling dramatically.. like driving a car with a badly slipping clutch or a high stall race  hi stall non lock torque  converter around town.

 Subject to more accruate data, your current prop is close, may need a little more rake if cruises with bow high. possible little more cupping also.
 I would not go getting another prop till permatrim removed, engine set right height or close as possible, slightly on the deep side..and good data.
 Get rid of the drag and that prop could/ likely be THE prop


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Ok ill get the boat out onto the water with some accurate data of speed etc with permatrim off.  

Hull is a 2000 surtees 5.5 thats overall length.  Measured properly its 5m. it is a slightly different design then the more modern ones.  

I am fairly sure I cant go any higher with outboard without having the cav plate out of the water and have spent quite some time working on this to get it.  Ill take some photos if I can of the water coverage and I am open to trying to raise it if its not right.



The 115 suzuki is def heavier then the honda I had by about 30kg an does ride abit nose high off the plane.

the Boat has heaps more grunt with the suzuki then with the honda.
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Had the boat out last night in harbour and did some playing around.

2POB  50l fuel and chilly bin otherwise pretty empty.
Played around with the trim a whole lot - and conditions were perfect
2.4K  RPM  6.2 Knots
3.0K  RPM  9Knots-13Knots planing
3.5k  RPM  15knots
4.0k  RPM  19 knots
4.1k  RPM  21 knots
4.5K RPM  24 knots
6kK RPM 33-34 Knots

I spend some time at the back of the boat and much to my suprise (deep sigh and shakes head at himself) The Permatrim is actually lower then I thought and looks to be to low, ie Can barely see it at while on plane covered i spraying racing water.  So I will now go and pull engine up another hole (approx 1 inch) and see how that goes.  

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The Permatrim is actually lower then I thought and looks to be to low, ie Can barely see it at while on plane covered i spraying racing water.  So I will now go and pull engine up another hole (approx 1 inch) and see how that goes.  

So as not to assume anything...the following data is with the engine low correct?


2.4K  RPM  6.2 Knots    displacement
3.0K  RPM  9Knots-13Knots    planing displacement to semi displacement..At this point while trying to climb the bow wave fuel consumption generally becomes rather high.
3.5k  RPM  15knots   29% slip
4.0k  RPM  19 knots  21%
4.1k  RPM  21 knots   15%
4.5K RPM  24 knots   11.7%
6kK RPM 33-34 Knots  7.4%

The current prop (14X19) with the engine height and permatrim as it is is pretty close to what it should be.. maybe another 1/4" in diameter..

The effective total weight on the water (including extra drag from permatrim plus low leg, is around 1400kg..
 So assuming the engine has not been lifted and estimated actual total weight on the water, full tank, gear etc, I would estimate that the permatrim plus low leg could be adding, ineffect  150 to 200kg..
 If my estimate is ball park correct, removing the p/trim , bring up the engine to correct height, you would pick up significant saving in fuel when cruising on the plain.

A WoT around 36/37.5 knts at targeted 5400/ 5300 rpms @ around 5% slip with about a 21" pitch prop.. 
with a 14 1/4 or 14 1/2" diameter
 
A 4000 rpm cruise around 21/ 22knts
This is subject to good data once the engine has been lifted and permatrim removed.
 Fresh good data with the current prop will give accrute data to select the actual corect prop once lifted.
 Also note that you should expect the WoT rpms to go over the 6000 rpms.
 So if could borrow a   
14 1/2"x 19 or 
14X 21" or 
14 1/4"X 21 or
14 1/2 X 21
 would be so much better

2.8k RMP 7-8 knots its thirsty AS
... trying to get some better economy lower down... would like to run around 7 knots and not drink my fuel so the RPM needs to be 2.2-2.4max
Currently thats abut 56% slip.. which to me seems very high.
 I dont have my head around displanement hulls other than the very basic concepts
A reduction in drag (p/trim, low leg gone) 
Increase in pitch to 21 and diameter (subject to data once engine / p/trim done) should bring your 7 to 8 knt rpms down to around the 1950 to 2050 rpms

Ideally I would like to run around 7 knots and not drink my fuel so the RPM needs to be 2.2-2.4max.

 Which I would estimate at cruise you will pick up a good 20% increase in economy, and think significantly so at troll speed




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So as not to assume anything...the following data is with the engine low correct?


2.4K  RPM  6.2 Knots    displacement
3.0K  RPM  9Knots-13Knots    planing displacement to semi displacement..At this point while trying to climb the bow wave fuel consumption generally becomes rather high.
3.5k  RPM  15knots   29% slip
4.0k  RPM  19 knots  21%
4.1k  RPM  21 knots   15%
4.5K RPM  24 knots   11.7%
6kK RPM 33-34 Knots  7.4%

Yes the above data is with the engine set to low and I will put up some more data once its been lifted.

I have pulled up some data on weight of the standard hull from an old review of the 5.5 workmate (https://www.surteesboats.com/assets/Uploads/Surtees-5.5-Workmate-Review-Dive-New-Zealand.pdf)   

The bare hull is 450kg apparently, and with a 2 stroke on trailer 750kg so prob 810-820kg with my 4 stroke on trailer plus all the bits and bobs, I am not sure how this relates to on water weight

The current prop (14X19) with the engine height and permatrim as it is is pretty close to what it should be.. maybe another 1/4" in diameter..



The effective total weight on the water (including extra drag from permatrim plus low leg, is around 1400kg..
 So assuming the engine has not been lifted and estimated actual total weight on the water, full tank, gear etc, I would estimate that the permatrim plus low leg could be adding, ineffect  150 to 200kg..
 If my estimate is ball park correct, removing the p/trim , bring up the engine to correct height, you would pick up significant saving in fuel when cruising on the plain.


The permatrim was on outboard when I purchased it and its held on with many bolts.  Will look into removing it and what it will take to fix up the holes.

A WoT around 36/37.5 knts at targeted 5400/ 5300 rpms @ around 5% slip with about a 21" pitch prop.. 
with a 14 1/4 or 14 1/2" diameter
 
A 4000 rpm cruise around 21/ 22knts
This is subject to good data once the engine has been lifted and permatrim removed.
 Fresh good data with the current prop will give accrute data to select the actual correct prop once lifted.
 Also note that you should expect the WoT rpms to go over the 6000 rpms.

6000RPM is the max for the engine so what would I theoretically do to stop this? I suspect a cable adjustment.


The rest below is a work in progress?
 So if could borrow a   
14 1/2"x 19 or 
14X 21" or 
14 1/4"X 21 or
14 1/2 X 21
 would be so much better

2.8k RMP 7-8 knots its thirsty AS
... trying to get some better economy lower down... would like to run around 7 knots and not drink my fuel so the RPM needs to be 2.2-2.4max
Currently thats abut 56% slip.. which to me seems very high.
 I dont have my head around displanement hulls other than the very basic concepts
A reduction in drag (p/trim, low leg gone) 
Increase in pitch to 21 and diameter (subject to data once engine / p/trim done) should bring your 7 to 8 knt rpms down to around the 1950 to 2050 rpms

Ideally I would like to run around 7 knots and not drink my fuel so the RPM needs to be 2.2-2.4max.

 Which I would estimate at cruise you will pick up a good 20% increase in economy, and think significantly so at troll speed




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For lower speeds some extra diameter (blade area) on the prop usually helps.Trouble is that this will bring down your WOT rpm's which you want to keep close to that 6000 rpm figure. Drop that too much and you will notice that the effective torque drops away and the engine won't hold it's rpms as well at any speed (like driving in a gear too high). Painful when trolling in waves as the rpm and speed go all over the show.

If you keep 2 props and one of them just for trolling days then you might try a 14.5 x 19, or if you lift the engine you might even be able to try a 14 x 20 or 14.25 x 20. Gut feeling says the 14.5 x 19. You will still be fighting the boat trying to climb onto the plane at 7knts etc but you may get your prop slip down and find a happier spot in the rpm range for the engine.

BUT - I don't know that the DF115 gearcase will take more than a 14" prop. Need to check. If not, you may want to try a 4 blade. Get one with some stern lift if possible and maybe something like a 13.75 x 19. It probably won't do anything for your cruise or WOT speeds but might help the trolling with the extra blade area and stern lift. You could also try adding a it more weight to the bow (shift your mate there to try) to see if that helps.
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Interesting looking at SOLAS NZ for suzuki they only do 13"7/8.  guess its close to 14.  having more weight at the front helps to a point.  My suspicion is it will change a lot once Ive lifted it.

For DF 90, DF 115 and DF 140, please use 15 tooth spline rubber hub propeller. For other SD propellers, please use 13 tooth spline rubber hub
propeller

Stainless Steel 3 Blade 
HR TITAN 3
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15 TOOTH SPLINE

Price:
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Have you got trim tabs?If not and you are moving the engine, what about adding some negative trim wedges to help lower the bow?
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6000RPM is the max for the engine so what would I theoretically do to stop this? I suspect a cable adjustment.

 Retailers tend to set up.. prop for max rpms.. they dont have a full load on gear, etc...which in 'therory' puts the WoT rpms around mid range.
 If they propped to mid range, then the end user loads up the boat you will loss all thwe reserve power/ torque at a given rpmd. plus load the power head causing long term slow death of the wengine.
 This is what the outboard manufactures recommend.

 You will also note that 'trials' 'reveiws' are generally propped for best speeds, economy, and a boat that is not fully loaded.. Its called marketing propaganda.

 The rating on this boat is max 115hp...which means it should be well powered.. If propped well it will still have enough resever power in lower cruise rpms so that when in chop, you set the throttle and sit back... rather than work the throttle in an min or under powered boat.. working the throttle uses huge amounts of fuel... setting the throttle you you less than normal cruise.
On a well powered trailer boat you should not need to have a P/trim to lift the stern...and if the bow still doesnt come down, a prop with a little more rake levers it down

Do not even consider spending any money on a prop at this point in time...
use the existing if cant borrow as suggested above.. or look on trade me.. even a cheap alloy for $50 or $70

At this piont we are looking to get the engine height right.. generally this takes 2 goes.. 1st move is usually about 1/2" too high.. and tends to ventilate in a tight turn or in chop to easy...which usually means most tend to be set about 1/" to low.. mine is, sons is.

 I ran into issues with my 115 johnny.. I calc needs a 14 1/4 or 14 1/2 prop which they do not make...
So I went to a stiletto 4 blade think a 13 1/4 or 13 1/2 which also has a lot more rake as most 4 blades do.
Min powered , leveled the boat right out, even on slow cruise (plain)
My cruise speed increased dramatically, the economy increased by over 45%...the performance in chop/ over wakes etc .. totally different hull.





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You have all forgotten one thing and this is an important constituent within these complex calculations requiring a theoretical mathematician... Wink

When Jake deploys a million lures, teasers, exciters, dredges, and bibbed minnows, his boat now has the transom dug in and the trolling speed has been reduced by 2.14 knots divided by pie arse root mean squared.

No prop will fix that!Ouch

Sorry Steps I couldn't resist. LOL
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Originally posted by Tagit Tagit wrote:

The prop isn't determining your fuel consumption. Your hull length is.

Jake there's some good advice & suggestions above, but Tagit's comment may be the most relevant.

I don't want to rain on your parade, but if improving efficiency/fuel consumption at displacement speeds is your sole aim, you are going to be disappointed if you spend a lot of time and money buying new props, adjusting engine height, permatrims, wedges - you might gain a bit but you won't beat the laws of physics.

You've probably already found the hull speed calculation on a hundred different websites (in case you haven't it's square root of WLL in feet x 1.34). The 1.34 can be increased a bit for different hull shapes - especially boats like cats, rowing skiffs, canoes, etc - but it works pretty well for both displacement launches and planing hulls in displacement mode.  

I get very similar revs/speed figures with a 6.1/150 Suzuki, (3-blade stainless) and have a similar sweet spot around 2200. 2400 is getting thirsty, 2600 scary. 

Trim the motor down to maximise your WLL, spend that $795 on unweighted symmetrical lures than work well at 6 knots, try keeping the tow point for your lures as high as possible & run your lures 1 wave further back than you might on a launch (outboard props are shallower & create more aeration)

Or take the advice from the guy in Jaws - you're going to need a bigger boat.
treat fish like fish
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Saltwater Fishing Reports
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Variety is the spice of life On one recent trip, the plan was to spend a... Read More >

22 Mar 2024
Saltwater Fishing Reports
Hauraki Gulf Fishing Report - 22/03/24

Fish where the fish are! Catching fish or just going fishing? I tackle this issue... Read More >

22 Mar 2024
Saltwater Fishing Reports
Inner Hauraki Gulf Fishing Report - 22/03/24

Thoughtful tactics required for better fish Over the course of each year the fishing varies,... Read More >

22 Mar 2024
Fishing bite times Fishing bite times

Major Bites

Minor Bites

Major Bites

Minor Bites