Learn more about using PropExpert to size propellers for powerboats!

  • Free webinar – PropExpert for Powerboats

    Aired Wednesday, April 5, 2017, 11:15AM EST
    Marine repair shops and outboard engine companies rely on PropExpert to size propellers as well as predict top speed, RPM, and fuel-usage for bow-riders, cabin cruisers, and other fast planing powerboats. Ensure your next project surpasses expectations with detailed considerations for shaft-driven, stern-drive or outboard driven vessels. Join HydroComp for this free online training seminar on propeller sizing for powerboats on Wednesday, April 5 at 11:15AM EST.
    Read more about PropExpert.

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April 10, 2017

Webinar: PropExpert for Powerboats

Considerations for using PropExpert to size propellers for stern-drives and outboards

HydroComp PropExpert is a program for the selection and analysis of propellers for motor yachts and workboats. While it was developed with the inboard propeller in mind, PropExpert can also be used for small high-speed powerboats – both recreational and commercial. This webinar provides guidance on some of the more common pitfalls and concerns.

What is a powerboat?
Powerboats are typically high-speed, planing vessels that are powered by stern-drives or outboard motors. They’re main objective is high-speed transit, they aren’t meant to carry cargo or for towing. Most small recreational boats are powerboats, but also some heavier commercial craft fall into this category as well.

What considerations are there for analyzing powerboats in PropExpert?
There are several important factors for accurate performance predictions for powerboats. These include: an accurate vessel weight, appropriate hull-propulsor interactions for stern-drives/outboards, proper identification of outboard engine RPM and gear ratio, and correction factors for outboard style propellers.

Importance of accurate vessel weight…
The vessel weight is a critical parameter for an accurate prediction, especially for powerboats that are often lightly loaded. There are very few parameters used by PropExpert to predict speed and other vessel properties, so the true vessel weight is a critical parameter. Because the boats are typically small and light, this means that passengers, fuel, gear, and other cargo are not negligible. Be sure to use the actual operating weight of the vessel underway. Remember that “light ship” and “gross/net tonnage” are not vessel weight.
In PropExert, we have provided a non-dimensional relationship of weight and length – the Ratio field. The weight-to-length ratio can provide some guidance when entering the vessel weight. A review of popular powerboats has shown that typical ratios for powerboats are between 100 and 300. The recreational boats tend to be near the lower end, ~100-200, while commercial vessels are on the upper end, ~200-300. Additional guidance is available in HydroComp Report 116.

Hull-propulsor interaction…
All propulsion systems interact with the hull and affect the performance in some way. PropExpert – being developed for inboard, shaft-driven propellers – automatically assumes that the propulsion system is inboard and populates all analyses with inboard estimates for hull-propulsor interaction. Powerboat propulsion systems are driven by stern-drives or outboard motors, therefore we must over-ride PropExpert’s estimates for inboard hull-propulsor interaction. The following values for wake-fraction, thrust deduction, and relative-rotative efficiency are recommended for stern-drives and outboard motors:

Shaft efficiency 0.97
Wake fraction 0.03
Thrust deduction 0.00
Relative-rotative efficiency 1.00

The hull-propulsor interaction coefficients are set from the Vessel Details page:

The engine considerations are fairly straight-forward. Outboard engines operate at high RPM’s (5k-7k RPM) and are often available in a range of rated RPMs and gear ratios. It is important to identify the rated RPM of the engine itself, but also the gear reduction built into the lower unit. PropExpert’s generic gas and diesel engine curves are appropriate in most cases, although the true engine power curve can be entered as well.

Outboard style propellers…
There are significant differences between inboard propellers and outboard style propellers. Outboard propellers have larger hubs to accommodate shaft splines and thru-hub exhaust. These propellers typically feature cup on the trailing edge, and may have face-camber and special pitch distributions. To model these propellers, we use the GawnAEW series in PropExpert as a basis, with some corrections to the Thrust (T) and Power (P) factors depending on the propeller type:

Stock propellers with flat-face
GawnAEW, No cup,
T factor = 0.99, P factor = 1.02

High-performance props with progressive-pitch
GawnAEW, Very light cup,
T factor = 0.93, P factor = 0.95

The T and P factors are entered from the Sizing page in PropExpert at the bottom of the propeller table.

Analysis and sizing…
After choosing the appropriate T and P factors, we can proceed to the propeller sizing. Often, we are just sizing for pitch. BAR is usually fixed for commercially available props off-the-shelf (i.e. 3 blade, 0.55; 4 blade, 0.65). Similarly, the gear ratio is dictated by the outboard or stern-drive – at best, we may have a few available ratios to choose from. Cup must be manually selected by the user.

Remember to check cavitation! Outboard/stern-drives are usually highly loaded, and excessive amounts of cavitation must be managed. For sub-cavitation propellers, try to keep the average cavitation below 20-25%. When the cavitation exceeds this range, the propeller begins to operate in a trans-cavitation regime with different thrust and torque behavior – this can sometimes make the sizing difficult to converge as PropExpert searches for the optimum propeller pitch and vessel top-speed. Fully-cavitating propellers may be difficult to size. In these cases, it may be best to run the sizing with Pitch = Keep and manually iterate through the pitch range to find the best combination of cavitation levels and top speed.

Also, the Calc sizing for = User-defined can also be used to help the sizing solution converge.

We would like to address some questions from participants at this time:

1) Can you save different Engine curves in PropExpert?
Yes, PropExpert includes built-in libraries for your engine data, as well as projects, clients, vessels, and propellers! To create and save an engine file, use the Library > My Engines > Create… command from the upper menu in PropExpert.

2) What is the Speed/power by: Average hull x 0.900 factor under vessel performance?
PropExpert uses vessel type, length, and weight parameters to estimate the thrust/power requirements using a mathematical relationship referred to as the “Average hull” formula. This is a “middle of the road” prediction that includes the hull, appendages (struts and rudders) and seas drag typical for a boat of that weight and length. The multiplier is used to characterize a vessel that may be hydrodynamically worse or better than the “average”. We have found that the Average Hull formula is most-often conservative, hence the default multiplier is 0.900. This multiplier is usually left as 0.900 unless there is seatrial data or a similar vessel that can be used to estimate a more characteristic multiplier for a particular vessel. There is a webinar on this subject which should be helpful: PropExpert – Using “Prior Trial” with PropExpert.

3) On an outboard propeller, is DAR a non-issue?
We are typically limited to a given blade area for a particular off-the-shelf outboard propeller model. As a result, it is unlikely that you would size the “Blade area ratio”. Be sure that you use the value that is for that propeller model.

4) How do you handle 3-4 outboard engine fit out?
As long as all outboards are the same, it is very easy to model 3 or 4 outboard engines on one vessel. The propeller count on the Vessel page determines the number of propulsors/engines on the vessel, so if you have 4 outboard motors, you would set this count to be 4.

5) How do I assess the additional drag created from the stern-drive or outboard leg and lower unit?
The hydrodynamic forces on the submerged portion of the outboard or stern-drive is not explicitly considered as a resistance/drag. Instead, the net thrust for the entire unit is corrected using appropriate thrust deduction factors and propeller T and P factors.

6) What is the maximum cavitation allowed?
For subcavitating propellers, which are very well-behaved, the cavitation should be kept below 15%. However, outboard style propeller often carry higher levels of cavitation. In cases where the propeller cavitation is excessive (trans-cavitating or fully-cavitating condition), PropExpert will appropriately correct the thrust and power. Once cavitation approaches 50% or higher, the calculations are no longer reliable.

7) How do you model contra-rotating propeller sets?
Contra-rotating stern-drives are not inherently supported by PropExpert, but can be modelled using this same procedure with some additional corrections and assumptions. First, we assume that the engine power will be equally split between the forward and aft propellers. To do this in PropExpert, you setup your propeller count as two propellers per unit (i.e. Propeller count = 2 for a single CRP shaftline) and enter half the engine rated power. Next, we must account for the efficiency improvement in CRP’s. This efficiency improvement for CRP’s (typically 8%) is due to the recovery of rotational energy, therefore it is reasonable to model this improved efficiency by increasing the relative-rotating efficiency from 1.00 to something like 1.08 (to represent an improvement of 8%). Lastly, when sizing the propeller, remember that forward propeller is always a lower pitch than the aft propeller. We represent the CRP system by sizing the an “effective propeller” that represents an the numerical average for pitch of the forward and aft propellers. As a result, the effective pitch will correspond to the forward advance of the CRP system, even though the forward propeller would be slightly higher pitched and the aft propeller slightly lower than this effective number.
More information on this subject is available through our Knowledgebase.

8) How do contra-rotating propeller sets differ on a pod?
The effect of podded drive bodies, like the contribution of the submerged outboard leg and lower units, are modeled as corrections to the hull-propulsor interaction coefficients and the T and P correction factors for the propeller performance. The process is largely the same, with some different recommendations for wake fraction and thrust deduction.
The following are typical values that can be used for an under-hull tractor unit (e.g., IPS):
Wake fraction = 0.00
Thrust deduction = 0.03
And these are typical for an under-hull pusher unit (e.g., Zeus):
Wake fraction = 0.06
Thrust deduction = 0.05
Some guidance on this subject is available through the Knowledgebase.

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