MODIFIED 13 Suzuki DF60

Optimizing the performance of Boston Whaler boats
Zarati
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MODIFIED 13 Suzuki DF60

Postby Zarati » Sat Apr 11, 2020 10:43 pm

After 22 years of dreaming about Boston Whalers, I treated myself to [a customized and modified 13-footer] for my 45th Birthday. This [customized and modified 13] has been extensively refurbished and modified [see images below]. It has been re-powered a Suzuki DF-60a with a PowerTech SCB4 10.375-inch-diameter, 15-pitch right-hand four-bladed propeller, and fitted with smart tabs--basically a spring-loaded trim tabs. A custom fuel tank is mounted up front where the bow locker used to be.

With four aboard--my wife and two kids--the fuel tank 1/4-FULL, a cooler, and full gear aboard, the DF-60a accelerated to over 6,200-RPM without any problem and produced a speed-over-ground per GPS of 32 to 33-MPH in very calm water. Acceleration was great. There was no bow rise. The boat just raises out of the water, like an inboard ski boat.

I would like the top boat speed to be 37 to 40-MPH SOG, and I feel like it is possible.

The propeller manufacturer says the SCB4 propeller has "excellent cornering, [acceleration from a standing start], and porpoise control' and is a "preferred choice on inflatable[boats] and [boats with ] flat-bottomed hulls."

To me the [10.375-inch] diameter of the SCB4 propeller seems small.

Q1: will a different four-blade propeller with more [blade] area [produce the improvement in boat speed desired]?

Zarati
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SUPER SPORT 13 Suzuki DF60

Postby Zarati » Sat Apr 11, 2020 11:11 pm

Re the DF-60: I didn't put it on there. It weighs 229-lbs (the same as a 40-HP) which I believe is not more than an old 40-HP three-cylinder two-stroke-power-cycle engine weighed. If the boat can go 40-MPH and if I and determine that to be unsafe, then I just wont' go that fast.

The gear ratio on this motor is 2.27:1.

Q2: what effect does engine gear ratio have on engine speed?

Q3: would changing to a propeller with 16-pitch [from the present 15-pitch] compensate for that gear ratio on the DF60a being different than a 2:1 ratio?

ASIDE: some views of the boat on a trailer.
13-1a.jpg
Fig. 1. stern.
13-1a.jpg (17.24 KiB) Viewed 7909 times


13-2a.jpg
Fig. 2. starboard side.
13-2a.jpg (20.97 KiB) Viewed 7909 times


13-3a.jpg
FIg. 3. modified interior.
13-3a.jpg (44.19 KiB) Viewed 7909 times

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sun Apr 12, 2020 8:17 am

Q4: When the fuel tank is 1/4-FULL, how many gallons of gasoline are in the tank?

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sun Apr 12, 2020 8:25 am

Zarati wrote:Q2: what effect does engine gear ratio have on engine speed?


Since you cannot change the engine's gear ratio, I would not worry about its affect on engine speed.

The engine gear ratio couples the propeller load to the engine crankshaft. The higher the numerical ratio of the gear ratio, the more slowly the propeller shaft rotates but with higher torque.

At maximum throttle the engine accelerates to the highest speed that allows it to turn the propeller load, and then the engine stops accelerating and operates at that speed.

In your desire to reach 40-MPH, the engine gear ratio is not going to be a factor. Boat speed is a function of power-to-weight ratio, not engine gearing. The propeller is selected to suit the engine power at whatever gear ratio the engine has.

Zarati wrote:Q3: would changing to a propeller with 16-pitch [from the present 15-pitch] compensate for that gear ratio on the DF60a being different than a 2:1 ratio?


You are not trying to "compensate" for a "gear ratio." You are selecting a propeller that allows the engine to accelerate to its maximum rated speed at full throttle, whatever gear ratio the engine has.

Changing to a propeller with 1-inch higher pitch, that is to a 16-pitch from a 15-pitch, will increase the load on the engine. This will likely result in the engine being unable to accelerate to 6,200-RPM as it did with the 15-pitch propeller load. Exactly what speed the engine will be able to accelerate to is difficult to predict with precision, but a rule of thumb would suggest perhaps about 6,000-RPM.

Predicting boat speed at RPM=6000 and PITCH=16 and SLIP =10 produces MPH=36.

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sun Apr 12, 2020 8:38 am

You report the following performance:
RPM=6200
RATIO=2.27
PITCH=15
MPH=33

From this data the SLIP can be calculated as 0.2 or 20-percent.

A propeller SLIP of 20-percent is higher than desired at the full-throttle boat speed. Usually a properly suited propeller will produce a SLIP in the range of 7 to 10-percent.

On that basis, a different propeller should be tested. And likely a propeller with larger diameter.

If a different propeller produced 10-percent slip and was also 15-pitch, the predicted boat speed would be 37-MPH, still short of your goal of 40-MPH. However, if the new propeller has larger diameter, it may not be possible for the engine to accelerate to the same 6,200-RPM due to increased load. The engine reacts to increased load by accelerating to a lower speed at full throttle.

To reach 40-MPH with propeller with 10-percent SLIP will require a propeller pitch of 17-pitch with the engine still accelerating to 6.200-RPM. However, since you report that the engine could only accelerate to 6,200-RPM with a 15-pitch propeller, it seems unlikely that the engine will have enough power to turn a 17-pitch to the same speed.

There is one element of the engine speed that is not clear to me:

Q5: When the engine speed accelerated to 6,200-RPM, was the engine speed being limited by the engine itself to protect it from damage?

Q6: what is the manufacturer's recommended maximum engine speed?

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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sun Apr 12, 2020 8:46 am

In small boats like a SUPER SPORT 13, the total weight of the boat will be very much changed by the weight of passengers and gear carried aboard. As a result, the maximum boat speed will be very affected by the crew weight and other added weight.

For selecting a propeller to produce optimum outcomes, small and light boats may need to be fitted with one propeller to use when only one person is aboard, and with a different propeller to use when four people are aboard.

The modifications to the 13-foot hull shown in your photographs look like they have added additional weight. Your expectation that a boat speed of 40-MPH will be possible with the same boat weight as you tested, that is, four people, cooler, gear, fuel, and so on, may not be attainable with a 60-HP engine.

El Rollo
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Re: SUPER SPORT 13 Suzuki DF60

Postby El Rollo » Sun Apr 12, 2020 11:36 am

There is a ton of info on this site about getting the most performance out of the classic 13 Whaler.

I hate to burst your bubble, but that particular hull was simply not designed or intended for high speeds.

There are several people on here that I'm sure will offer much better first hand experience about the infamous hook in the hull.

Generally a three-blade propeller will be faster on most given situations.

If you do prefer a four-blade prop, you may want to look at the SCD-4 line of Powertech propellers. They have a stern-lifting characteristic which may benefit your particular set up.

i've heard of people getting some good speeds out a 13-footer, and plenty who say it can be done, but it can also be dicey in terms of handling.

Your boat seems to be be set up nicely, and I'm sure you'll get plenty of good use and compliments with it.

I wouldn't get too hung up on hitting 40-MPH, you may easily be able to get there, or you may not. Enjoy.

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Phil T
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Re: SUPER SPORT 13 Suzuki DF60

Postby Phil T » Sun Apr 12, 2020 7:07 pm

Q7: What is the boat's performance when running solo with a light load?

Q8: Is the desired WOT speed desired when the boat is empty or loaded with four?

If you want an increase in speed a four-blade prop is a hinderance since they are designed for holding power.

Prop selection is 80% science and 20% nuance.

Answers to the above questions will provide information necessary to give you specific prop make/model and size and engine height recommendations.

ASIDE: It would be a visual improvement if you replaced the existing decals with OEM originals available from http://www.magicbrushsign.com, the sole OEM vendor.
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Zarati
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Re: SUPER SPORT 13 Suzuki DF60

Postby Zarati » Mon Apr 13, 2020 12:38 am

jimh wrote:Q4: When the fuel tank is 1/4-FULL, how many gallons of gasoline are in the tank?

I believe capacity is about 9-gallons. Boat is brand new to me. I plan to fill it up soon which should give me an indication of capacity. I'm sure there are 75-lbs up front between the tank, amplifier, battery (a lightweight unit like race cars use) and the platform.

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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Mon Apr 13, 2020 4:20 am

If capacity were 9-gallons and the tank were 1/4-FULL then it would have 2.25-gallons. The weight of 2.25-gallons of fuel is not particularly significant in having an effect on boat performance, as that is only about 14-lbs.

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Mon Apr 13, 2020 4:21 am

Performance data for unmodified 13-foot hulls may not be applicable to the modified 13-foot hull under discussion here.

Q9: What is the maximum propeller diameter possible for a DF60a engine?

To answer my own question, I found this Suzuki propeller chart for the DF60 engine:

SuzukiPropeller.png
Fig. 4. Suzuki propeller chart
SuzukiPropeller.png (37.57 KiB) Viewed 7803 times


[UPDATE: while the above chart is accurate for the DF60, it is not accurate for the DF60a. See the appropriate chart below in a follow-up post, and ignore the advice that follows.]

It looks like a 14-inch diameter propeller can be used. On that basis, using a propeller with a 10.375-inch diameter seems very odd and very small diameter. Whoever made the choice of a 10.375-inch propeller likely did not appreciate the large propeller aperture on the Suzuki DF60a. The SMALLEST Suzuki propeller for this engine has a 13-inch diameter.

A more appropriate propeller might be the steel three-blade 16-pitch or 14-pitch, both 13-1/4-inch diameter or the aluminum 17-pitch or 15-pitch.

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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Mon Apr 13, 2020 7:28 am

jimh wrote:Q6: what is the manufacturer's recommended maximum engine speed?


To answer my own question, the recommended full throttle range for a Suzuki DF60a engine is 5,300 to 6,300-RPM

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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Mon Apr 13, 2020 7:35 am

Zarati wrote:ASIDE: some views of the boat on a trailer.


The stern view in Figure 1 is so badly illuminated that no details of the engine mounting height or the trim tabs are visible. If you want to illustrate the engine mounting, produce an image with better lighting that more clearly shows the engine mounting height and the trim tab installation. The image you provided has the entire stern in deep shadow.

Also, with all the added weight in the bow, and with the hull having an intentional hook to limit bow rise, I am surprised that trim tabs were needed to keep the bow down during transition to plane or while running on plane.

Q10: where are the seats for four people? I only see in Figure 2 what appear to be one thwart seat across the cockpit at the helm.

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Re: SUPER SPORT 13 Suzuki DF60

Postby biggiefl » Mon Apr 13, 2020 12:03 pm

I am probably the closest you will get to dialing that boat in as I have an almost similar setup.

I have the same hull design with a three-cylinder DF40. I have the engine mounted all the way up( last hole) and it is fine. Your engine mounting height is way too low.

I am running a 17-pitch OMC Rapture SS propeller. Best boat speed is about 38-MPH with bimini up and just above the 5,800-RPM redline (but not hitting any limiter.) I know she can handle that RPM as the 50-HP redlines at 6,300-RPM and does not explode.

I had the propeller re-pitched to roughly a 16-pitch as the 17-pitch was giving me about 5 500 to 5,600-RPM, but when you have three or four people aboard, engine speed dropped to the low 5,000-range.

Like said above, the 60-HP is excessive, and you will never be able to utilize it on a 13-foot hull as it was not designed for speed. That boat was designed to run great with a 25-HP.

At 38-MPH on calm water and shooting a five-foot or six-foot-tall rooster tail, my boat would tend to chine walk. With it being a tri-hull it has more like a wiggle butt as the stern is wiggling back and forth. It was not very comfortable and the next pass I knelt on the deck to lower the center of gravity.

The main advantage with the 60-HP for your boat is the higher weight you have compared to my boat. With four people and 75-lbs plus fuel up front, my boat would probably be limited to 30 to 32-MPH.

I also agree that a three-blade propeller is much better for speed. I would think you should be able to hit 40 with a light load and correct propeller, as I could probably do so myself if I lowered the bimini and added a smidge more pitch to the propeller.
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Zarati
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Re: SUPER SPORT 13 Suzuki DF60

Postby Zarati » Sat Apr 18, 2020 3:47 am

jimh wrote:....looks like a 14-inch diameter propeller can be used...appropriate propeller might be the steel three-blade 16-pitch or 14-pitch, both 13-1/4-inch diameter or the aluminum 17-pitch or 15-pitch.


The engine is DF-60a, a lighter-weight version with a much smaller [gear case] than the DF60. The DF60a is the top of the 30 to 60-HP model line and the DF-60 is the bottom of the larger model line 60 to 70-HP, which uses a much larger propeller.

Thanks for [the link to the Suzuki propeller catalouge] which shows the DF-60a has a range of 11.125 to [11.75-inches].

[Below I added the more appropriate chart for the DF-60a propeller choices--jimh]
suzukiPropellerChart.png
Suzuki propeller chart for DF-60a
suzukiPropellerChart.png (65.23 KiB) Viewed 7676 times

Zarati
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Re: SUPER SPORT 13 Suzuki DF60

Postby Zarati » Sat Apr 18, 2020 4:19 am

Q5: When the engine speed accelerated to 6,200-RPM, was the engine speed being limited by the engine itself to protect it from damage?

A5: With people in the boat [the engine acceleration] seemed to stop on its own. I'm not sure where the actual limiter cuts in; the maximum engine speed is rated for 6,300-RPM.

Q6: what is the manufacturer's recommended maximum engine speed?

A6: The recommended full throttle range for a Suzuki DF60a engine is 5,300 to 6,300-RPM

Q7: What is the boat's performance when running solo with a light load?

A7: Not Sure yet. I have not had it out with just me or just me and the kids. Hope to find out this weekend.

Q8: Is the desired WOT speed desired when the boat is empty or loaded with four?

A8: I'm not after just speed. I feel there is speed on the table that I want to utilize.As has been pointed out there is no point in chasing speed with this hull design, but I feel that with some tweaking, I should be able to get more top speed and maintain a strong low speed. A speed of 32-MPH seems low to me. If I only end up with 36-MPH then so be it.


I took a straight edge from the bottom of hull and my [anti-ventilation plate NOT cavitation plate] is about 1-inch below the hull. The engine is mounted one-hole-up from the lowest mounting position. I will try raising the engine mounting.

Biggiefl: Thanks for the comments on setup. I was debating if I should just go all the way up as that would put [anti-ventilation plate NOT cavitation plate] about 0.75-inch above the bottom of hull and about one-foot back from hull when trimmed level. I'll try that. I have a feeling the boat speed will increase,but the engine will hit the rev limiter. One change at a time. Height first, and test with light load, then I'll change the prop.

Q9: What is the maximum propeller diameter possible for a DF60a engine?

A9: 11 3/8 to [11.75]-inches. The DF60 uses larger propellers. DF60a (which is what I have) uses smaller propellers.

Q10: where are the seats for four people? I only see in Figure 2 what appear to be one thwart seat across the cockpit at the helm.

A10: the boat has a swing out seat bracket that folds out of the way. One seat is in front of console. Another is in its forward position but can also be mounted on the port side in the same location.

seats.jpg
Fold out seat on starboard, with another seat on bow platform.
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smartTabs.jpg
Smart tabs on transom of modified 13-footer.
smartTabs.jpg (13.7 KiB) Viewed 7671 times
The smart tabs may not be needed, I will see if they can easily remove and try it both ways.


I need to research what hook means as in this hull design that everyone keeps referencing. I noticed a change from sharp to round hull at the midpoint of the outer sponsons. Have not really analyzed the rest of the hull yet.

Zarati
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Re: SUPER SPORT 13 Suzuki DF60

Postby Zarati » Sat Apr 18, 2020 4:24 am

swingSeat_.jpg
Forward seating on modified 13-footer
swingSeat_.jpg (14.34 KiB) Viewed 7666 times

Zarati
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Re: SUPER SPORT 13 Suzuki DF60

Postby Zarati » Sat Apr 18, 2020 4:29 am

biggiefl wrote:I am probably the closest you will get to dialing that boat in as I have an almost similar setup.

I have the same hull design with a three-cylinder DF40. I have the engine mounted all the way up( last hole) and it is fine. Your engine mounting height is way too low.


Thanks for your comments. I was wavering on how high to go but was leaning towards trying it all the way up based on my measurements. I'll report back once I do.

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sat Apr 18, 2020 11:33 am

I apologize for the confusion in the propeller aperture diameter. I didn't see the separate listing of propellers for the DF60a model. Now going back to the first question in this thread:

Zarati wrote:Q1: will a different four-blade propeller with more [blade] area [produce the improvement in boat speed desired]?


With the notion that actual maximum propeller diameter for the DF60a engine is about 11.75-inches, the present propeller with 10.375-inch diameter now seems more appropriate, but there is still another 1.375-inch increase in diameter possible. Using a larger diameter propeller should tend to reduce the SLIP factor, which (as calculated from your data) was abnormally high at 20-percent SLIP.

Re total boat weight: the added upholstered seats in the forward part of the boat, their mounts, and the swing out mechanism have added even more weight to the hull. You really have a much heavier boat than the original 13-foot open skiff from Boston Whaler.

Re using 60-HP which exceeds the original rating: this modified 13-footer boat may need 60-HP to propel its heavier hull and four people onto plane and give some reserve power.

Re removing smart tabs: a much simpler test of the influence of the smart tabs will be to just temporarily draw them upward with a bungee-cord or some similar lashing.

jimh
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Re: SUPER SPORT 13 Suzuki DF60

Postby jimh » Sat Apr 18, 2020 11:53 am

Zarati wrote:I need to research what hook means....


The classic 13-foot Boston Whaler skiff hull bottom was designed with a hook at the transom. The term "hook" means there is a noticeable downward curve at the transom. A hook is incorporated into the hull in order to tend to force down the bow at planing speeds. In effect, the hook is a built-in trim tab mechanism.

The opposite of hook is "rocker". A hull bottom with rocker has an upward curving shape at the transom.

There is no guarantee exactly what bottom shape profile you will find on your highly modified 13-foot hull. In a quest for more speed or at least more control over bow rise, some 13-foot owners have faired the hull bottom at the stern to remove the hook. You can easily see if this has been done by inspecting the hull bottom shape along the center keel line from the transom forward several feet. If the hull bottom is straight and true, the hook has been removed. If the hull bottom curves downward somewhat toward the transom, the hook is still there.

The hook was mentioned as a speed reducing effect because as boat speed increases, the influence of the hook becomes greater, and the bow will tend to be forced downward. Trimming down the bow is generally not desired at high boat speeds. Usually for high boat speeds the bow is to be carried clear of the water. For best planing at high speeds, the only part of the hull bottom in the water will be just the stern area, perhaps only a quarter of the hull (or less). This type running attitude will be impossible with a hull bottom that has an intentional hook at the transom.

Below I show an example of a 15-foot hull running on plane with very little hull (or wetted surface) in the water.

Image
MISCHIEF 15 with 60-HP engine running on plane with reduced wetted surface. The 15-foot hull has no hook in the running surface at the transom.

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Re: MODIFIED 13 Suzuki DF60

Postby jimh » Sat Apr 18, 2020 12:04 pm

As mentioned above (in immediately prior post), a hull with a hook at the transom has, in effect, a built-in trim tab. This effect, if it exists in your modified hull, may eliminate the need for smart tabs.

Also, the added weight in the bow on your modified 13-footer will also tend to keep down the bow rise when on plane. That will also mitigate the need for smart tabs.

Generally generating a force with trim tabs that will lower the bow introduces drag. Adding drag retards boat speed. In the case of smart tabs, when the boat is on plane at speed, the spring expansion force pushing down the tabs is probably not sufficient to force them into the water stream very far, if at all, so they may have little to no effect on boat trim at higher planing speeds.