Guardian 18 Propeller Ventilation

Optimizing the performance of Boston Whaler boats
GuardianRC
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Guardian 18 Propeller Ventilation

Postby GuardianRC » Thu Sep 08, 2016 9:03 am

Hi everyone, I am a new member over in the United Kingdom.

I picked up a c.1990 Guardian 18 hull last year and spent a bit of time putting a few bits on so that i could use it--it has been abused!

I'm using a Mercury or Mariner 150 V6 two-stroke with a LASER II 19-pitch propeller. I've finally got it out on the water and i'm struggling with a few things.

As you apply some throttle from idle, the boat starts to accelerate but then the engine will rev completely disproportionally to the gain in speed (like a car with the clutch worn out). I manage it by reducing the throttle and then re-opening slowly, a process which could happen a few times before you're planing.

I'm also only achieving around 25-nautical-miles-per-hour flat outl was expecting to hit more than 34-nautical-miles-per-hour.

I don't have an RPM tachometer currently.

Any help or advice would be greatly appreciated!

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jimh
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Re: Guardian 18 Propeller Ventilation

Postby jimh » Thu Sep 08, 2016 12:21 pm

Does the LASER II propeller have the hub vents completely closed? There are plastic inserts for the three hub vents that are available in various configurations, from completely sealed to progressively larger openings. If these vents are open then the propeller bite or grip is spoiled (intentionally) during acceleration onto plane. The result is a relationship between engine speed and boat speed that is not linear, as you have described.

What is the engine mounting height? (Please use the method of describing engine mounting height explained in the pinned article at the top of this forum; see

http://continuouswave.com/forum/viewtopic.php?f=7&t=739

Are there any appendages mounted to the hull, such as a SONAR transducer, a water pick-up for a live well, or other additions on the running surface of the hull that might disrupt the smooth flow of water off the hull and into the engine gear case and propeller?

jimh
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Re: Guardian 18 Propeller Ventilation

Postby jimh » Thu Sep 08, 2016 12:36 pm

A boat speed of 25-nautical-miles-per-hour is equivalent to 28.9-MPH, which is not very good performance for an 18-foot Boston Whaler classic hull with 150-HP. As suggested, a boat speed of 34-nautical-miles-per-hour or 39-MPH would be more in line with expectations.

Boat speed for planing-hull boats is generally related to the horsepower-to-weight ratio. To account for the low top speed of your boat we would first look at this ratio. The GUARDIAN series for the 18-foot hull is built with heavier lay-up, so we can anticipate the boat weighs more than the usual recreational-grade 18-footer. There is also a substantial frame and hard top, adding more weight (and air resistance). The auxiliary engine is more added weight. Thus we might have to reduce expectations of boat speed somewhat from the usual experience with the lighter recreational boats without added hard tops and auxiliary engines

To produce its rated horsepower, a two-stroke-power-cycle outboard engine must be able to accelerate to an engine speed in the recommended full-throttle engine speed range. If the engine cannot accelerate to this speed range it will not produce its rated power output. Since there is no measurement of engine speed available, we can only surmise that perhaps the engine speed is not reaching the proper full-throttle speed range. The typical engine speed range for a Mercury V6 150-HP engine at full throttle should be 5,000 to 5,600-RPM.

I don't have literature for MARINER brand engines. For a classic Mercury 150-HP XR6 engine, the gear ratio was a rather tall 1.87:1. With a 19-pitch propeller, the engine turning 5,500-RPM and assuming SLIP was 10, the boat speed should be in the region of 48-MPH or 41.6-nautical-miles-per-hour.

Since your performance is no where near this benchmark, I recommend you invest in an engine tachometer. Evaluation of propeller performance without accurate measurement of engine speed at full throttle is going to be highly speculative and prone to guesswork.

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Phil T
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Re: Guardian 18 Propeller Ventilation

Postby Phil T » Thu Sep 08, 2016 1:51 pm

I think the hub insert inside the prop is stripped. This is a replaceable part.

Before you replace it, I second the installation of a tachometer. Your hull with that engine should reach +/- 40-43 mph with a light load. I would retest the boat and measure speed at WOT with a GPS.
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GuardianRC
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Re: Guardian 18 Propeller Ventilation

Postby GuardianRC » Thu Sep 08, 2016 3:58 pm

Thank you for the replies!

The speed was measured with GPS.

There is a fish finder transducer on the transom, however it is very small, I will remove it and re test .

I wasn't aware of the slots in the laser props, I'll check out what configuration I have going.

I will try and get hold of a tacho and see what's going on rev wise, I feel it's probably not revving out but can't be sure.

I'd had a thought it was a possible spun hub, however flat out I don't get the revving just low top end. Possible but I would have thought over revving would be likely at the same time.

I have a spare laser 2, in 21P, so likely over propping however may show the problem up as the hub.

jimh
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Re: Guardian 18 Propeller Ventilation

Postby jimh » Thu Sep 08, 2016 4:34 pm

If you have a second Mercury propeller that uses a FLO-TORQ hub kit, you can move the hub kit from the spare propeller to the in-use propeller.

If the LASER II propeller has the usual FLO-TORQ plastic hub, you can easily inspect the propeller and hub for damage. Pull off the propeller from the propeller shaft and take a close look at the hub. You should be able to see any evidence of the hub having become damaged and having allowed the propeller shaft to turn faster than the propeller.

I have not experienced a damaged FLO-TORQ hub myself. I suspect that damage occurs more easily if the engine exhaust is able to heat the plastic hub, which may occur more quickly if there is any deficiency in cooling water flow in the engine, or if the engine is being run with a heavy load and is lugged down and producing higher exhaust temperatures. Once the plastic softens from too much heat I think it will be quite evident.

For more about Mercury propellers using the FLO-TORQ hub kits, see an earlier article on FLO-TORQ hub kits from the OLD FORUM at:

FLO-TORQ ADAPTERS
http://continuouswave.com/ubb/Forum4/HTML/005155.html

As I recall, one report of a failed FLO-TORQ hub included mention that removal of the partially melted hub was quite difficult due to deformation of the plastic. On that basis, I think you'd be able to detect a spun hub kit without too much ambiguity.

jimh
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Re: Guardian 18 Propeller Ventilation

Postby jimh » Thu Sep 08, 2016 4:49 pm

ASIDE: that boat looks great. Your restoration job has succeeded.

GuardianRC
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Re: Guardian 18 Propeller Ventilation

Postby GuardianRC » Sun Sep 11, 2016 12:36 pm

Thanks, again. I've had a look at my prop and it appears all of the vent holes are missing their bungs totally. Probably the cause of the ventilation! I'll order some and we can see.

The top speed would be unaffected by these. Am I right?

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Re: Guardian 18 Propeller Ventilation

Postby jimh » Sun Sep 11, 2016 1:25 pm

The theory of intentional ventilation of propellers at the exhaust hub with aerated water from exhaust gases is that as boat speed increases most or all of the exhaust gas will prefer to exit via the central hub, and exhaust gas venting through the hub vents will decrease to zero.

jimh
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Re: Guardian 18 Propeller Ventilation

Postby jimh » Sun Sep 11, 2016 1:27 pm

Use of a Mercury propeller with the Performance Vent System exhaust gas vents completely open is most likely the cause for the unusual behavior you described in which engine speed and boat speed seemed to be non-linearly related.

GuardianRC
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Re: Guardian 18 Propeller Ventilation

Postby GuardianRC » Sun Sep 11, 2016 3:43 pm

I have ordered the vent covers , so I will report back if they fix the concern. I will also get a tachometer to start to assess my very low top speed.