Repower: 1999 DAUNTLESS 15 Yamaha F70

[wetdogsoup]

Give me suggestions for a new propeller for my 1999 DAUNTLESS 15 with new Yamaha F70 engine.

BACKSTORY
I've just updated [my 1999 DAUNTLESS 15) to a new Yamaha F70 engine from its original Mercury 60-HP two-stroke-power-cycle engine. I need guidance for selection of a stainless steel propeller. At this point, the 20-hour break-in for the F70 is almost complete

TEST DATA
Propeller: a generic, worn, Aluminum propeller of 13-1/4-inch and 17-pitch [and unknown number of blades]

Test general conditions: 10-gallons of fuel, new bottom paint, trim tabs engaged, two aboard

Engine mounting height: [Moderator's note: please always describe engine mounting height as the number of holes the engine has been raised from lowest position. In this case, from Figure 1 we can see the engine is mounted two-holes-up.]

Fig. 1. Engine mounting height shown as "two-holes-up" from lowest position.

engineMountHeight.jpg (19.36 KiB) Viewed 11145 times

Engine trim: up a bit
Engine speed [assumed to be a full-throttle]: 5,100-RPM
Boat speed [assumed to be at full-throttle] 30-MPH

My analysis: [the engine speed at full throttle should] ideally be close to 6,300-RPM, the maximum engine speed limit. My goal:

• switch to a [three-bladed] stainless steel propeller; and
• increase engine speed [at full throttle] by 1,200-RPM.

Based on [other, not cited,] posts for [selecting a propeller for] the DAUNTLESS 15 with the F70, [those posts mention the chosen propeller pitch was] in the range of 14 to 15. Some of the propellers mentioned were:

1. Yamaha K Series Perf3 13-1/4 x 14-pitch GYT3B-V4-141
2. Yamaha K Series Turbo 1 13-1/4 x 15-pitch MAR-13215-TR-D02
3. PowerTech RED3 13-1/4 x (13, 14, or 15)3

Notes on the prices of the three propellers mentioned above from various sellers:
1--selling for $300 on eBay
2--selling for $250 on eBay
3--selling for $352

ASIDE
Thank you for the good information over the last 25 years of owning my Boston Whaler boat.

[jimh]

A very general rule of thumb for propeller influence on engine speed: a change in pitch of 2-inches will cause an engine speed change of 400 to 450-RPM.

Applying this rule in this case, then if pitch is reduced from 17 to 13 (4-inch reduction) then engine speed will increase 800 to 900-RPM.

Your mention of prior test results from other participants does not include the boat speed or engine speed data. Nor do you provide any boat speed data from your boat with the 60-HP engine. Knowing the anticipated boat speed can help in propeller selection.

[wetdogsoup]

Thanks Jim for your [estimation of the effect of pitch change on engine speed].

Unfortunately I didn't have GPS speed tracking ability six years ago when—the last time— the 1999 DAUNTLESS 15 ran with the Mercury 60 and no bottom paint.

Yes, the current Aluminum propeller is a three-blade.

Q1: Would there be an expected RPM change in switching from a to steel propeller from an aluminum propeller?

Otherwise, from your estimate it appears I should be looking at a 13-pitch propeller, which would lead me to either :

1) the PowerTech RED3 13-1/4 x13" New $352 or
2)Yamaha K Series Turbo1 13-1/4" x 13" P RH Rotation MAR-13213-TR-D0 New $420

If I'm on the right track, here's what [some unspecified AI response to an unspecified question] has to say about them:

While both are 13.25" diameter stainless steel propellers for 50–140 HP outboards, the Yamaha K Series Turbo 1 is a performance-oriented flagship with advanced venting for fast hole shots, whereas the PowerTech RED3 is a versatile, all-purpose model optimized for mid-range speed and fuel economy.

Powertech RED3 Propeller
Designed with moderate rake and cup to provide good lift and balanced performance. Expert testing on specific vessels found it delivered the best mid-range speed and fuel economy compared to other stainless options. It is often compared to the Rapture, Ballistic, and Stiletto propeller styles.

Yamaha K Series Turbo 1 Stainless Steel Propeller - 3 Blade - 13.25 Di
Features a specially-vented blade design that allows for a faster "time-to-plane" and better acceleration. Its progressive pitch and high rake angle provide enhanced bow lift, making it effective for medium-to-heavily loaded boats or high-performance pontoons.

So thinking I go with the Yam K series Turbo1?

Don

[jimh]

Q1: Would there be an expected RPM change in switching from a to steel propeller from an aluminum propeller?

Aluminum propellers typically will have more blade flexing than steel propellers will have, a result of the steel having greater tensile strength than aluminum. However, at the 70-HP level, an aluminum propeller probably does not have much blade flexing.

The blade flexing typically occurs during maximum loading of the propeller, which occurs, of course, when the engine is producing maximum power. The blade flexing reduces the effective pitch of the propeller, making the propeller load to be lower than if the blades had not flexed.

But exactly how this effect will occur at the 70-HP level is hard to say. But in general, a steel propeller will produce a greater load on the engine than an aluminum propeller, and that means the engine speed at full throttle would tend to be lower with a steel propeller than with an aluminum propeller. On the other hand, the boat speed may be better with a steel propeller.

The general advantage of a steel propeller over an aluminum propeller in terms of producing better boat performance is a result of the steel propeller being cast with thinner blades. The expectation is that a thinner blade will produce better boat speed than a thicker blade. To know exactly how much faster a particular boat and engine combination will run with a steel propeller than an aluminum propeller depends on the power level and the particular boat weight and hull design.

I would expect at the 70-HP power level and a boat like a DAUNTLESS 15 the difference in maximum boat speed obtained with a steel propeller compared to an aluminum propeller would be probably in the range of 1 to 3-MPH. The outcome really depends on the propellers used.

Comparisons of two propeller of different pitch and what engine speeds will result by using the rule of thumb depends entirely on the two propellers being essentially identical except for pitch. That means the diameter is the same and the propeller design in the same, which by that I mean both propellers are from the same family or model designation and, of course, from the same manufacturer.

The only recommendation I can give you regarding the propeller selection is to look for a propeller vendor that will sell you a propeller with the option to exchange it for another propeller if the initially purchased propeller is not satisfactory.

If you have found good first-hand accounts of identical boat and engine combinations as yours that have real measured data about boat speed and engine speed outcomes with a certain propeller, you can use that data as guidance. I cannot offer much further advice, as I don't have any first-hand data about a 1999 DAUNTLESS 15 and Yamaha F70 engine combination.

Because you have not provided any data about the engine speeds and boat speeds that were produced by using the several propellers you mention as being tried with similar boat and engine combinations, for me to assess or comment about those particular propellers and pick one to buy is impossible.