Boat has a left (port) leaning attitude

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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2018 at 9:34am
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Titanium
Titanium


Joined: 14 Oct 2013
Location: Franklin
Status: Offline
Points: 12849
Not sure a 115hp that is running OK is under-powered for boat. Weight including fuel+crew+gear is around 1500kg.
Prop is 14.5/19".
I've done the WOT/RPM which came in well within range attaining near 48kts@5000 RPM (did not like the fuel consumption though!)

 Re check your numbers.
 I think 1500kg would be a close estimate, maybe a little on the heavy side.

48knts (55mph) thats a real fast boat for its type...
Which would make , with 115 hp on the back only 690kg..
115hp 1500kg would have a WoT speed of around 38 mph (33knt)
Assuming this is a johnson/ evinrude and has a 2:1 ratio gearbox, 19 pitch  5000 rpms 55 mph give a why imposable slip of  minus 22%.. or in other word the boat is going way faster than the prop is turning... imposable.. ideal for general use boat like this around +5%
A boat similar size, 1200kg  with 150 hp on the back has a max speed approx 52 mph... and a 4000 rpm cruise speed around 38/39 mph... thats over powered.. 140 is well powered 115 is min powered and WoT around 42/43 mph

 
Assuming 1400 weight, 115 hp WoT would be 38/40 mph.
 @ a ideal slip of 5% on a 19" prop you would be doing that at around 4500 rpms, right on the bottom of the manufactures recommended, rather than 5000 mid point.
The 14.5" diameter prop would be close to spot on..
And a cruise at around 13% slip around 20/ 30 mph.

This would explain the permatrim...with that weight/ power and that stock type 19" prop it would be very slow on the hole shot, likely tend to cruise lowish in the stern....
 There would be no reserve power at low rpms.. cruise and below, so in chop be falling off the plain, have to speed up, big bang on next wave, throttle back to slow plain, fall off the plain...
 Throw in slow hole shot...
 Basically a little under powered and over propped.. ie a 16 or 17" prop would help, not solve, with a lot more rake to help lever the bow down. 
Your hole shot will improve.
This will drop your cruise speed (WoT stays close to the same with increase in WoT rpms) down to around 29/30 mph @4000 rpms.. this requires approx 72 hp.. ideally at this point you should have10 to 15% reserve power..around 80 hp @4000 rpms.
Unfortunately this means using higher rpms for the same speeds.. which means the engine volume it rturns over per distance increases, being under powered , the mixture doesnt lean much at all and the fuel consumption (L/mile) increases..( l/hr will reduce slightly or stay the same) and can be quite a bit.. if still cruise at the same speed..
 This is the essential part of sorting what power is requires... not the WoT specs.. its not a race boat.
 I dont have the rpm/hp graphs handy for the 115 johnson ATM to reference to..

Would not have thought such conditions could create such a attitude on boat

And with the current set up.. power prop etc, having little or no reserve power at cruise/ low rpms, winds will have a far greater effect on leaning the boat.
 And with most modern post general use, 70s plaining trailer hulls its not the hull design but rather the power available that has the greatest effect on its performance...

Anyway hope some of those number comments also help.


 

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Post Options Post Options   Likes (0) Likes(0)   Quote AdAstra Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2018 at 3:02pm
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Bronze
Bronze
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Joined: 09 Mar 2016
Location: Hawkes Bay
Status: Offline
Points: 94
Update on Left Leaning.
Last week armed with knowledge and advice from you all, I finally managed to get out on the water and conclude that the reason/s for this attitude were:

Trim
Wind and an ill-fitted hydrofoil/permatrim.

While at Mahia last week I had the opportunity to tinker around and found that the trim had a significant influence on this lean.
There was a brisk N'Easterblowing which replicated conditions down in Napier.
I managed to negate most of the lean by use of the trim.

Inspecting the foil I found it was fitted with one of the spacers missing. This caused the rear right hand side to 'droop' down.
This must have caused some irregularities to the way the boat rides over the water.

However, I cannot totally discount my somewhat over-weight son-in-law. He will not disclose his weight but I would estimate it to be around the 140kg mark which is 40 more than I.

I wish to thank all those who offered their opinions.

Cheers
Adastra  
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