The mystery of a proper prop and terrible performance

@Nidza , I'm curious what you mean about a deformed prop.

 

That chart in the magazine said prop is 18 x 19 inches.
So the boat's prop MIA, is too small

 

You are not taking blade area (expanded area ratio). into account. The propeller I'm using now is a Michigan Wheel DQ486.

The typical 3 blade prop has an EAR of 0.56. On an 18" wheel this produces a total blade area in the neighborhood of 142 square inches.

A 17" DQ486 has a blade area of a little better than 195 square inches. Remember the DQ486 is a 4 bladed wheel. Even though it's an inch less in diameter the blade surface area is considerably larger.

Insufficient blade area causes water pressure against the propeller blades to be too great. At a certain point the prop just slips and cavatates. This was my problem with the old propeller.

There are three main aspects of propeller sizing. Diameter, pitch and blade loading. The third one is often overlooked.

MIA

 

I did not realize you had already fixed the problem. but your original 14" prop was too small.

i think my 22 by 20 3 blade Michigan wheels are too big, I cant get the rpm's up past 3000 rpm. People change things over many years, my old boat is a 1970 Egg Harbor 37 with twin 265 HP engines and 3:1 velvet drive gear reduction. Boat weight 20,000 lbs and semi displacement style hull. Blade tips are two finger widths from hull, and I think other Eggs like mine used 17 or 18 inch props. I really think a PO messed with it.

 

Well, I haven't exactly fixed the problem yet but I'm moving in the right direction.

It's interesting that you mention the clearance between the prop and hull being only an inch or so on your Egg Harbor. Generally, clearance is about 15% of the prop diameter. There is a little "wiggle room" in that spec but I'd think that you should have somewhere in the neighborhood of 3 inches +.

It might be a good idea to see if you could have your propellers reworked. You could also run the engineering formulas that Dave Gerr presents in his book The Propeller Handbook. I don't trust those calculators that you see online. The only one I found that agreed with my calculations was from Victoria Propeller up in Canada.

www.vicprop.com

MIA

 

I got the boat as is with those props. A shop said they can rework them smaller.
I have been trying to fathom why a PO would do this, could it be he thought they were more efficient at very low speed-rpm.
I can get 2 fingers between blade tip and hull.
here they are from 2014, does 22 by 20 seem too big

 

I meant e.g. propeeler which would be designed as tilted on shaft to compensate the shaft angle. Due to being "offcenter" it would probably have to have some special weight distribution to compensate the disbalances and because of that I would expect it to be "deformed". The other problem would probably be not longitudinal thrust towards the shaft which would affect both bearing(s) and shaft. Many considerations, but tehcnology is long there, so I guess someone has explored it already. I am just curious.

 

Wow, it's been a year and I'm still working on this. Made some improvements but I'm not quite there. No need to read this long thread. Look at post #1 and then come back here.

So, she has a new v-drive and prop. 2.05:1 from Walter Gear and a 17X18 DQ 486 from Michigan Wheel. Replaced the mechanical distributor. New distributor really woke the 302 up.

At wide open throttle RPM with the old points and condenser system would only reach about 3,000 RPM. With the new distributor I get more bow rise and she'll reach about 3,300 RPM. You can feel the difference and she's throwing a mountain of water out the back. I tested her with the dinghy rack on the back but without the dinghy on the rack. The dink would add about 70 pounds.

She still can't quite get out of the hole. I can feel that she wants to go but there's just not quite enough thrust there.

So there is nothing left but trim tabs. I've done everything else. This will be a spring project.

My question is this, I'm comparing different trim tabs. I can certainly go with Bennett. I also noticed something new that's being made in Europe.

Zipwake | Dynamic Trim-Control System https://zipwake.com/

A friend was watching the back of the boat and commented that the swim platform was "going under" as the stern squatted at full throttle. He wondered if raising it up say 4 or 5 inches might reduce any drag that it was producing.

So many of you have been so good with your advise and suggestions I figured I'd go back to the well one more time. I'll post a few photos below.

Thanks in advance for any input you might have.

MIA

New distributor and tuning really improved performance.

With full fuel tanks and provisions she squats about 2 inches more at the stern than you see here.

She got a new paint job on the topsides too! Interlux Perfection roll and tip came out pretty good!

 

The boat was never weighed. How do we know the bootstripe is original?

Maybe it just kept creeping up.

Is the boat solid glass at 3/4" or is it a 70s balsa experiment? Either way sounds heavy.

Weigh it... if it is 9000 pounds; it would explain some of the issues, 11000 even more

boat is gorgeous, so hopefully no core issues, but boats tend to gain weight over years of ownership

 

Hi Fallguy good to hear from you and thanks for the compliment.

I don't know where I would get an accurate weight. Our Travelift doesn't have a scale and it's the only one around for quite a distance. I came up with the 8,000 pound estimate based on similar sized boats from that era that were comparably equipped that I could get information on. I could be off but at this point it seems to be academic. I don't think that it's too far off.

Regarding the fiberglass, yes it's solid. About 3/4" aft tapering to 3/8" forward. Topsides are 3/16" to 1/4". I had the seacock out under the engine and pulled a couple of thru hull transducers forward. Once I realized the hull was solid glass I replaced the transducers with the in hull type.

I remember having a conversation with Paul Ricelli. He explained that back in the 60's and early 70's builders didn't understand how fiberglass would hold up over time. Some of them overbuilt their hulls. As time passed this was considered too expensive and heavy. That's how we got to coring, at least that's how I remember it.

I stayed true to the original design. The flybridge was my idea but she had a molded fiberglass flybridge from the factory that was pretty heavy. I remember taking it off with a forklift. Again I'm guessing but I think the new one may be slightly lighter.

What I'm really fishing for is any insights that someone might have on those Zipwakes. They have a website but I don't see too much information on them yet. One way or the other it looks like trim tabs will be the last option. Fortunately I'm only looking to plane off in order to avoid weather or when transiting inlets where current is strong. 99% of the time I'll spend at 1,500 - 1,800 RPM and 6 to 8 knots. Would just like the ability to plane.

I'm not in a hurry and with the price of fuel, taking it easy saves a lot of money. I can't see gas getting a lot cheaper in the future.

Thanks for the reply,

MIA

 

This simply suggests that the vessel is too heavy for its length.
The length-displacement ratio is low, and the result of such is a vessel has a very high prismatic hump resistance and is characterised by extreme squat/trim.

If the power installed is insufficient at the hump, she will never get over and past the hump.

So the biggest clue is this:

Without knowing what the actual as she is sitting in the water weight is, you will be chasing a never ending circle.
On boats, especially high speed boats - weight kills!

So, knowing what the weight of the vessel is, in the condition you are running is key to unravelling the issue(s).

 

Thanks for the reply Ad Hoc. Everything you say is true. For me though getting her weighed isn't practical right now. If you scroll up to post #152 you'll see a performance chart and a link to a review of a Silverton 26. That boat is almost identical to mine in design. 1/2' longer, same beam, same horsepower. It's said to weigh 6,925 pounds. According to the specs the 26 carries 20 more gallons of fuel and had three people aboard when tested. I was alone the other day. When I restored my little cruiser I stayed true to the original design with the exception of the flybridge. Could she be heavier than 7,000 pounds? I suppose, but not by too much. I think that it's more about where the weight is in this case.

I took care to save a little weight where I could. I knew I'd be adding that swim platform and the kicker. The original transom needed to be reinforced. To compensate for those changes, I moved her batteries (3) into the main cabin below the sole (use AGM's) and moved her water tanks forward as well. The original mufflers were these cast iron monsters. I must have saved 100-150 pounds or more back there by installing those Centek's. On the other hand, the kicker, swim platform and the dinghy add about 350 pounds to the transom. This weight isn't in the boat, it's hanging off the transom. It's not surprising that she'd need a little push up.

I used Dave Gerr's Propeller Handbook to determine the appropriate prop and gearing. When run at full throttle she is smooth and exhibits no vibration. She's just not quite getting over the hump. I always knew that trim tabs would be the last option, if necessary. Thus my question, especially about those Zipwake's.

Bennett's technical people suggested 24" x 9" or 24" X 12" tabs. That's an option as well.

One last thing. The objective here is to have the option of getting this boat on plane. I'm heading for the ocean and will need to deal with more currents, tides and inlets. I'll still spend 99% of my time at trawler speeds.

I can't remember if you and I ever communicated when I was restoring my boat years ago. I'll just post a couple of photos below. This is my retirement hobby. I don't gamble, don't drink much and I stopped chasing women a long time ago.
This was taken 12 years ago.
This was taken last summer.


Thanks for taking the time to reply Ad Hoc.

MIA

 

If you are at all serious, after changing the gearing and prop, you'd at least remove all the excess weights to see what happens.

Moving water forward in a planing hull is not wise. I am actually moving my tanks closer to cob, com. Excess weight forward in a planing hull seems like the magic, but if the boat is too heavy forward; it takes a lot of power. And the Silverton doesn't have lots to spare. Sorry.

For your next efforts, rather than testing 'worst case', test best case. Pump out b/w, run on less than 5 gallons f/w or zero, and get rid of everything that ain't tied down, even the kicker. Inside the salon, I see what appears to be a LOT of mdf or ply panels. Those add up fast and they are forward of com. Keep a tally of estimated weights. You can even pull the table it appears. I would unless she's more than a one bolt..

Your closed cooling system, what did that add in weight? Where is that tank?

Wouldn't you like to know what happens?

My small skiff had a rather poor hole shot. The boat was originally designed for a two stroke 60hp. I had a 50hp 4 stroke. I removed all non-essential gear from the back of my boat and weighed it. It was 83 pounds of stuff in a 16 foot skiff. I fish Lake Superior, added back a 37 pound kicker, for a net change of nearly 50 pounds and it made a difference.

I'd be willing to bet you can remove 100 pounds from just behind kitchen cabinets in pots and pans and kitchen gear and foodstuffs.

To pretend it doesn't matter because this is how you intend to run is silly. Fact finding is all it is...

I've seen quite a few boats that happily operate at 8 kts. They tend to get loaded full of junk.

Make an estimate of the weights on the dock. I'd be willing to bet you can put a thousand pounds on the dock. And I bet the boat weighs 9000-11000 pounds currrent state. When I look in the salon; it looks like a thousand pounds of cabinetry...

I'd put money on a bet that you have two stern anchors onboard...the power is too little for too much weight. In a test, those get removed..

For the fuel, filling full is not even typical case! Typical is average, so that should be half full for the test, or less to see what happens.

All the best.

ps-I am not being a hotshot, I have some boat weight issues of my own and this winter need to remodel the salon to shed 100 pounds...

 

I understand what you are saying Fallguy. I didn't have much weight in her. Just me on board. I was getting ready to pull her out for the season. Flybridge seats were removed as was the dinghy. 150 pounds right there. Water tanks empty but full fuel tanks. No provisions or traveling supplies.

If I strip everything out maybe she'll plane. I don't use the boat that way though. I travel on her. So there will typically be extra fuel, maybe 10 gallons of drinking water, about 35 gallons of fresh water in the tanks, provisions, a couple of Honda 1000 watt generators, even a little 2.5 HP outboard for the dinghy. I try to travel light. The two generators and the 2.5 outboard weigh about 100 pounds combined.

I moved the batteries and water tanks forward. Both are right on the centerline. The batteries are about 3 feet aft of the exact for and aft center point. Water tanks are right at the center point. She's not nose heavy.

Maybe we're both right in different ways. The photo below will give you an idea as to how far off the OE prop was on this boat. The small prop was original equipment. Stamped 14x16 and dated 1973 from Michigan Wheel. When I worked with Walter Gear I had the serial number off the v-drive. They verified it was made way back then. A 1.46 to 1 unit. The man at Walter commented "I have no idea why that drive is in there, that's a ski boat drive. You need a 2:05 : 1 drive and a new prop." After working with Michigan Wheel that's what I did. She runs much better than she did when I relaunched her in 2014. Even at semi displacement speeds she's much more efficient. So all this effort hasn't been wasted.



There is only one thing left to do. If the trim tabs don't work out, I'll just remove them and sell them as they'll be practically new. I'm still trying to get more information on that Zipwake system. We'll see how it goes.

Thanks a lot for your advice,

MIA

 

It is just a test...if weighing her is out of the question; next best thing...how many 26' boats with a 200hp engine and 1/2" average glass hulls are going to plane with a 150 pound dinghy?

I surmise the boat's weight in some database is a clear miss.

I'm sitting in my boat atm, regretting every piece of 3/4" lumber in here. Wishing it was all 3/8"...