Which prop (3 or 4-blade) for a 18.5 ft Alicraft 18 Norwind with Mariner 90 2T

GreedyMuffin

Petty Officer 3rd Class
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Jul 1, 2016
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97
Hi!

I had a Rana 16 Line, but I bought an Alicraft 18 Norwind a few weeks ago, and the same Mariner 90HP 2T (From 2001) will be pushing the boat.
I want an aluminium prop due to lower prices.

The Alicraft is 5,65m long, 2,30m beam, 500KG dry-weight without people, gas, engine etc. This is the official numbers from the brochure that I found (It was from the 70s, a picture is added, but it's on Norwegian, but you can see the names and numbers).


The hull seems to be good, some said that it's a 24 degree hull, could that be correct?

A few pictures, not my boat. The hull on the inboard and outboard is excatly the same.

I've been looking at a 15P Spitfire prop from Quicksilver, I have not tested the engine, RPM etc. as the boat is still 3 hours away, and the engine is in my garden.

I reckon the top speed is between 33 and 36 knots based on others (30-32 with a 70-75 HP 2T, 33 with an F80 first gen).

I want to have the best economical speed, top speed is not important, will never run it on WOT. I do fish/troll, so the three "speeds" I will be using is either 1-3 knots, 5-8 knots or 23-26 knots. Trolling, calm cruising or my go to speed for hurrying. :)
This is why I really want a four-blade prop.

Thanks! :)

Important notes for engine: Max recommended RPM is between 5000-5500, 2,33 gear ratio, 90HP two-stroke
 

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alldodge

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Like all prop questions we need a baseline to provide information. The boat needs to be run with a prop and determine what is WOT in order to decide which way to go. While many folks want a 4 blade prop, the main reason is not based on testing, its based mostly on the expectation of something great happening. IMO you need to wait and give it a good run before deciding what needs to change
 

Sea Rider

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Need to test current prop if OB has one on a wot spin as usually loaded, report back max wot rpm achieved, will need a tach for this test. Once knowing wot numbers can go for a prop maximization to reach 5500 wot revs. Once there can throttle at the rpm/speed you like most.

Don't know why boaters wants to go blindly for 4 blade props when haven't tested 3 blades props with less pitch if in need to pull wot revs up, it's a night day difference.

Happy Boating
 

GreedyMuffin

Petty Officer 3rd Class
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Jul 1, 2016
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I have an 19P three blade prop that I can test with, but it's a few months away.

I want a fourblade due to the "big" boat compared to the engine HP(?).

I have a tac, no issues. :)
 

alldodge

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The 4 blade prop takes more HP to turn, and provides more lift. Lift can be a good thing IF you need more lift, but can cause other issues like proposing if you don't need more lift.
 

Sea Rider

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"I want a four blade due to the "big" boat compared to the engine HP".

Which is the max HP rated for that boat ? if underpowered, you'll be surprised what a 3 blade less pitch can do for your combo. It's much better to test current 3 blade prop on a wot spin as usually loaded and depending on wot revs achieved modify pitch at a prop shop if possible and if hub is OK or go for a prop maximization than start blindly with a 4 blade prop as a starter prop. All Dodge is correct on his post.

Happy Boating
 

GreedyMuffin

Petty Officer 3rd Class
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Jul 1, 2016
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97
Hi!

My boat with that engine is not underpowered it seems like. Most were run with a 70-75HP and gave 30-32 knots, a 90 of my size and make will probably do 35 or so.

I've got a 13x19x3 I can test, but when I need to buy a new prop, that will be the final prop as I can't buy a four blade after having tested the three blade. I can't afford that.

But I can test the 19P before buying a new prop. I reckon my max RPM will be around 4500, and it will use alot of time getting on plane. But we'll see!

Thank you all for your help so far, a newbie like me really appreciates it! :)
 

Sea Rider

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You're welcome,

Post wot rpm test run with that 19 as currently loaded on flat calm no wind water cond, will know how far off from max wot rpm range that 90 is. Afterwords can go for a lees prop pitch tu pull wot revs to max range.

Happy Boating
 

GreedyMuffin

Petty Officer 3rd Class
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Jul 1, 2016
Messages
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Hello!

I got a blackmax 13.75x15x3 today, used once. For 28% of the price compared to a new one. The one I picked up was in a new condition.

Really happy with that!:)

Probably going to be the perfect prop as I have something to go on when we are 6 persons + gear in the boat.

Will update once tested. :)
 

jimmbo

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May 24, 2004
Messages
12,944
An 18 1/2 ft boat with a 90hp, a 3 blade 15" will probably be the correct prop. A 4 blade will be adding about 30%(allowing for a slightly smaller dia) more blade area and unless there is a decrease in pitch will heavily load the engine. But as mentioned by others find out what the current prop delivers. Myself, I wish I could find a 2 blade prop for my boat/engine, but I like speed...
 

GreedyMuffin

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Hello! Tested the boat today. First with the 15P prop and with calm and sunny weather, then later with windy/dark weather..

Alicraft 18.5 ft (565x230 - 500GK + engine etc)

With the 15P i reached the RPM-limit and 28 knots.

Then I tested with the 19P, 5200-5300 RPM and 30 knots in some rough sea + a good amount of wind towards us. So on a calm, nice day. 32-33 knots might be possible..

The acceleration with the 15P was rather insane. From idle/neutral to full plane was 3 sec(!!), and with the 19P, I got 6-7 sec. Still very good. The boat is very front-heavy.

So stupid of me buying that 15P, but I did not really think that a 19P would be this good for this boat.

Overall exited over the boat and the engine (which ran butter smooth all the time, nothing to complain on my maiden voyage with a "new" boat and a "new" engine)

I was on plane all the way down to 14-15 knots. Worked superb. About 20-21 knots with 3800 RPM.

The speed was monitored with GPS, and RPM with a RPM-gauge from Mercury.
 

Sea Rider

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With 15 pitch as loaded, did OB achieved max 5500 wot revs ? did rev limiter kicked in if having one ?
If with the 19 prop achieved 5200-5300 rpm as loaded OB is running at middle of the 5k-5.5K min-max wot rpm range.

As you have experienced less pitch achieves higher wot revs, tad less top end speed, but faster hole shot
compared to a higher pitch which achieves less wot rpm, slight more top end speed, but sacrifying faster hole shot. So it's a matter of choosing the prop performance you're comfortable with. For me top hole shot than tad more top end speed as loaded.

If both stated wot rpm were achieved while heavily loaded, that OB will over rev when lightly passenger loaded, so watch tach...

Happy Boating
 

GreedyMuffin

Petty Officer 3rd Class
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With the 15P I reached the RPM limit. 6000 RPM, Insane hole shot, 3 sec from idle to full on plane, compared to 6-7 secs with the 19P, but on the other hand, only 28 knots, and with 19P I got a steady 30 with rough sea and wind. And the RPM was perfect on the 19P. It might drop to 5000 when heavily loaded.

My normal load is usually 2-3 people with gear. I only testet two people + a little gear today.

So the 15P was a waste of money as it hit the RPM limiter ASAP. Will sell it, hopefully to the same price as I paid for it as well.
 

Sea Rider

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According to Mariner it's same 5000-5500 wot rpm range as the 2 strokes Merc 90. No need to full throttle, middle to 3/4 throttle will suffice to plane without insane hole shot. Just for the record, test that 15 with 3 souls and gear at wot, report max rpm OB achieved, will be less than 6100.

Happy Boating
 

GreedyMuffin

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Jul 1, 2016
Messages
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I can test, but the engine does not seem to be alright.. Starts up good, running good. But with my boat people get 35 knots with 90HPs, and around 30-31 with 70 and 75 HPs. I barerly get 30..

My engine does not seem to use much fuel. After two hours ( or so) of playing at various speed, it only drank around 11-12L. Very much was at lower RPMs, but it was quite a while. Is it possible that my engine is trash? :/

The compression is at 7.5 bar on each sylinder, so they are excatly the same.. But the speed is not good, but the fuel usage is great.

I have 3 guys saying 35 knots with good weather and such with 90HP. The boat is does not have a strong V-hull, and it is very light for its size.

The engine starts on the second try everytime, does not bug down, idles great. Can do low-RPM trolling without any issues etc. Oil pump is working. When I first had it, the oil was red in the tubes, I added Quicksilver's green oil, and now the oil is very dark. So it is definetly mixing.

I talked to my nearby Merc. dealer, and they did not recommend to remove the oil injection, they said it was good and no need on these newer engines. On the older ones, they would.
 

Sea Rider

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Does OB have back or over water splashes on transom, if so, lower leg is dragging the combo a bit. Check. OB needs to run top before playing with props. If it's a used OB, a good overdose of Carbon Guard, Power Tuner through carb's throats while OB is on is a good maner to restore lost power, follow can instructions. After that install new well gapped plugs and go for a wot run to clear product from cylinders.

Happy Boating
 

GreedyMuffin

Petty Officer 3rd Class
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Jul 1, 2016
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The gearhouse plate is mounted 1 cm over the bottom of the boat. As far as I know, that is pretty optimal with a three blade alu prop?

I can try to test Carbon guard or power tuner trough the carbs. Does the engine need to be in gear, or is idle and fast idle good enough?

Will need to check if I got some sort of thing in Norway.

By the way. The gearoil, plugs, impeller and such was changed by a dealership in 2016, before I got the engine.
 
Last edited:

walleyehed

Admiral
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Jun 29, 2003
Messages
6,767
Haven't seen it mentioned but prop condition is extremely important. Trailing edge cup can wear without much visual indication. Engine mount height can change speed substantially. Can you confirm gear ratio? I believe it's 2.33-1 so it's going to use more pitch than a 2-1 ratio because it turns slower.
P/N or model of prop is more important when testing also. You're talking in apples and oranges here. I might also mention a boat will always be slower on flat water. As an official prop tester for many manufacturers, I will tell you first-hand the manufacturers do not want test data from flat water.
 

GreedyMuffin

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The first prop was brand new, the other one was in good condition as well. I do not think it is the prop.

The ratio is indeed 2,33:1.
 

walleyehed

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You need to target 5750-5800 to set it up for trolling, load hauling and the speeds you mentioned. If you'd get a P/N off of those props I can tell you which way to go.
Are you at sea level?
 
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